Light-emitting element, display module, lighting module, light-emitting device, display device, electronic appliance, and lighting device

ABSTRACT

An object of one embodiment of the present invention is to provide a multicolor light-emitting element that utilizes fluorescence and phosphorescence and is advantageous for practical application. The light-emitting element has a stacked-layer structure of a first light-emitting layer containing a host material and a fluorescent substance, a separation layer containing a substance having a hole-transport property and a substance having an electron-transport property, and a second light-emitting layer containing two kinds of organic compounds that form an exciplex and a substance that can convert triplet excitation energy into luminescence. Note that a light-emitting element in which light emitted from the first light-emitting layer has an emission spectrum peak on the shorter wavelength side than an emission spectrum peak of the second light-emitting layer is more effective.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.14/548,947, filed Nov. 20, 2014, now allowed, which claims the benefitof foreign priority applications filed in Japan as Serial No.2013-249486 on Dec. 2, 2013, Serial No. 2014-097803 on May 9, 2014, andSerial No. 2014-180913 on Sep. 5, 2014, all of which are incorporated byreference.

TECHNICAL FIELD

One embodiment of the present invention relates to a light-emittingelement containing an organic compound as a light-emitting substance, adisplay module, a lighting module, a display device, a light-emittingdevice, an electronic appliance, and a lighting device.

Note that one embodiment of the present invention is not limited to theabove technical field. The technical field of one embodiment of theinvention disclosed in this specification and the like relates to anobject, a method, or a manufacturing method. In addition, one embodimentof the present invention relates to a process, a machine, manufacture,or a composition of matter. Specifically, examples of the technicalfield of one embodiment of the present invention disclosed in thisspecification include a semiconductor device, a display device, a liquidcrystal display device, a light-emitting device, a lighting device, apower storage device, a storage device, a method of driving any of them,and a method of manufacturing any of them.

BACKGROUND ART

In recent years, research and development of a light-emitting element(organic EL element) that uses an organic compound and utilizeselectroluminescence (EL) have been actively promoted. In a basicstructure of such a light-emitting element, an organic compound layercontaining a light-emitting substance (an EL layer) is interposedbetween a pair of electrodes. By applying voltage to the element, lightcan be emitted from the light-emitting substance.

The light-emitting element is a self-luminous element and thus hasadvantages that the visibility of a pixel is higher than that of aliquid crystal display and that a backlight is not needed, and isconsidered suitable as a flat panel display element. In addition, it isalso a great advantage that a display including the light-emittingelement can be fabricated as a thin and lightweight display and has veryfast response speed.

The light-emitting element can provide planar light emission. Thisfeature is difficult to obtain with point light sources typified byincandescent lamps and LEDs or linear light sources typified byfluorescent lamps. Thus, the light-emitting element has great potentialas a light source applicable to a lighting device and the like.

In such an organic EL element, electrons from a cathode and holes froman anode are injected into an EL layer. By recombination of the injectedelectrons and holes, the organic compound having a light-emittingproperty is excited and provides light emission.

The excited state of an organic compound can be a singlet excited stateor a triplet excited state, and light emission from the singlet excitedstate (S*) is referred to as fluorescence, and light emission from thetriplet excited state (T*) is referred to as phosphorescence. Thestatistical generation ratio of the excited states in the light-emittingelement is considered to be S*:T*=1:3.

In a compound that emits light from the singlet excited state(hereinafter, referred to as fluorescent substance), at roomtemperature, generally phosphorescence is not observed while onlyfluorescence is observed. Therefore, the internal quantum efficiency(the ratio of generated photons to injected carriers) of alight-emitting element using a fluorescent substance is assumed to havea theoretical limit of 25% based on the ratio of S* to T* that is 1:3.

In contrast, in a compound that emits light from the triplet excitedstate (hereinafter, referred to as a phosphorescent compound),phosphorescence can be observed at normal temperature. Since intersystemcrossing (transfer of excitation energy from the singlet excited stateto the triplet excited state) easily occurs in a phosphorescentcompound, the internal quantum efficiency can be increased to 100% intheory. That is, a light-emitting element using a phosphorescentsubstance can have higher emission efficiency than a light-emittingelement using a fluorescent substance. For this reason, light-emittingelements using phosphorescent compounds are now under active developmentin order to obtain highly efficient light-emitting elements.

A white light-emitting element disclosed in Patent Document 1 includes alight-emitting region containing a plurality of kinds of light-emittingdopants that emit phosphorescence. An element disclosed in PatentDocument 2 includes an intermediate layer (a charge-generation layer)between a fluorescent layer and a phosphorescent layer (i.e., theelement is what is called a tandem element).

REFERENCE

Patent Document 1: Japanese Translation of PCT International ApplicationNo. 2004-522276

Patent Document 2: Japanese Published Patent Application No. 2006-120689

DISCLOSURE OF INVENTION

As a multicolor light-emitting element typified by a whitelight-emitting element, as in Patent Document 2, an element including afluorescent layer (a layer emitting light with a short wavelength), aphosphorescent layer (a layer emitting light with a long wavelength),and an intermediate layer (a charge-generation layer) between thefluorescent layer and the phosphorescent layer has been developed andpartly put into practical application. This element has a structure inwhich two light-emitting elements are connected in series with theintermediate layer sandwiched therebetween.

In this structure, fluorescence is used as light with a short wavelengththat has a problem in the lifetime and phosphorescence is used as lightwith a long wavelength. With this structure, a multicolor light-emittingelement with stable characteristics can be obtained though its emissionefficiency is lower than that of an element in which phosphorescence isused as light with a long wavelength and light with a short wavelength.

The multicolor light-emitting element with the above-described structurehas high reliability and is suitable for practical application; on theother hand, a larger number of films need to be formed to obtain onelight-emitting element, which hinders the practical application of thelight-emitting element.

There are some reasons for providing the intermediate layer between thephosphorescent layer and the fluorescent layer in the multicolorelement. One of the reasons is for preventing quenching ofphosphorescence caused by the fluorescent layer.

In the fluorescent layer, a substance having a condensed aromatic ring(especially, a condensed aromatic hydrocarbon ring) skeleton, typifiedby anthracene, is often used as a host material. This is because whenthe substance having a condensed aromatic ring skeleton is used as ahost material of the fluorescent layer, a light-emitting element withfavorable characteristics can be stably obtained; on the other hand, asubstance having a condensed aromatic ring skeleton generally has adisadvantage of low triplet level. Thus, in the case where thefluorescent layer is fog led in contact with a phosphorescent layer, thetriplet excitation energy generated in the phosphorescent layer istransferred to the triplet level of the host material of the fluorescentlayer to be quenched. Since a triplet exciton has a long lifetime, thediffusion length of the exciton is long and excitation energy generatedin the phosphorescent layer as well as excitation energy generated atthe interface between the fluorescent layer and the phosphorescent layerare quenched by the host material of the fluorescent layer. Thus, asignificant reduction in emission efficiency is caused.

The above-described problems are solved by using a host material withhigh triple excitation energy for the fluorescent layer. In that case,however, the singlet excitation energy of the host material is higherthan the triplet excitation energy, so that an energy difference betweenthe singlet excitation energy of the host material and the singletexcitation energy of a fluorescent dopant becomes too large and thusenergy is not sufficiently transferred from the host material to thefluorescent dopant. This results in insufficient emission efficiency inthe fluorescent layer. As a result, non-radiative decay of the hostmaterial is accelerated to degrade the characteristics (especially,lifetime) of the element in some cases. When the singlet excitationenergy of the host material is higher, the HOMO-LUMO gap of the hostmaterial is necessarily large. This leads to an increase in drivevoltage.

In view of the above, an object of one embodiment of the presentinvention is to provide a novel light-emitting element. Another objectof one embodiment of the present invention is to provide a multicolorlight-emitting element that utilizes fluorescence and phosphorescenceand is advantageous for practical application. Another object of oneembodiment of the present invention is to provide a multicolorlight-emitting element that utilizes fluorescence and phosphorescence,has a small number of fabrication steps owing to a relatively smallnumber of layers to be formed, and is advantageous for practicalapplication.

Another object of one embodiment of the present invention is to providea multicolor light-emitting element that utilizes fluorescence andphosphorescence and has high emission efficiency.

Another object of one embodiment of the present invention is to providea multicolor light-emitting element that utilizes fluorescence andphosphorescence, has a relatively small number of layers to be formed,is advantageous for practical application, and has high emissionefficiency. Another object of one embodiment of the present invention isto provide a novel light-emitting element.

Another object of one embodiment of the present invention is to providea display module, a lighting module, a light-emitting device, a displaydevice, an electronic appliance, and a lighting device that can befabricated at low cost by using the light-emitting element.

Another object of one embodiment of the present invention is to providea display module, a lighting module, a light-emitting device, a displaydevice, an electronic appliance, and a lighting device that have reducedpower consumption by using the light-emitting element.

It is only necessary that at least one of the above-described objects beachieved in the present invention.

The above-described objects can be achieved by a light-emitting elementthat has a stacked-layer structure of a first light-emitting layercontaining a host material and a fluorescent substance, a separationlayer, and a second light-emitting layer containing two kinds of organiccompounds that form an exciplex and a substance that can convert tripletexcitation energy into luminescence. Note that a light-emitting elementin which the first light-emitting layer has an emission spectrum peak onthe shorter wavelength side than the second light-emitting layer moreeffectively achieves the above-described objects.

One embodiment of the present invention is a light-emitting elementincluding a pair of electrodes and an EL layer positioned between thepair of electrodes. The EL layer includes at least a firstlight-emitting layer, a second light-emitting layer, and a separationlayer positioned between the first and second light-emitting layers. Anemission spectrum of the first light-emitting layer is in a shorterwavelength region than an emission spectrum of the second light-emittinglayer. The first light-emitting layer contains at least a fluorescentsubstance and a host material. The second light-emitting layer containsat least a substance capable of converting triplet excitation energyinto light emission, a first organic compound, and a second organiccompound. The first organic compound and the second organic compoundform an exciplex.

Another embodiment of the present invention is a light-emitting elementhaving the above-described structure in which the separation layercontains a substance having a hole-transport property and a substancehaving an electron-transport property.

Another embodiment of the present invention is a light-emitting elementhaving the above-described structure in which the substance having ahole-transport property and the substance having an electron-transportproperty forms a second exciplex.

Another embodiment of the present invention is a light-emitting elementhaving the above-described structure in which the thickness of theseparation layer is greater than 0 nm and less than or equal to 20 nm.

Another embodiment of the present invention is a light-emitting elementhaving the above-described structure in which the thickness of theseparation layer is greater than or equal to 1 nm and less than or equalto 10 nm.

Another embodiment of the present invention is a light-emitting elementhaving the above-described structure in which a combination of thesubstance having a hole-transport property and the substance having anelectron-transport property is the same as a combination of the firstorganic compound and the second organic compound.

Another embodiment of the present invention is a light-emitting elementhaving the above-described structure in which energy is transferred fromthe first exciplex to the substance capable of converting tripletexcitation energy into light emission.

Another embodiment of the present invention is a light-emitting elementhaving the above-described structure in which the singlet excited levelof the host material is higher than the singlet excited level of thefluorescent substance, and the triplet excited level of the hostmaterial is lower than the triplet excited level of the fluorescentsubstance.

Another embodiment of the present invention is a light-emitting elementhaving the above-described structure in which the triplet excited levelof the host material is lower than the triplet excited level of thesubstance having a hole-transport property and the triplet excited levelof the substance having an electron-transport property.

Another embodiment of the present invention is a light-emitting elementhaving the above-described structure in which the host material is anorganic compound having a condensed aromatic ring skeleton.

Another embodiment of the present invention is a light-emitting elementhaving the above-described structure in which the host material is anorganic compound having an anthracene skeleton.

Another embodiment of the present invention is a light-emitting elementhaving the above-described structure in which the host material is anorganic compound having an anthracene skeleton, and the fluorescentsubstance is an organic compound having a pyrene skeleton.

Another embodiment of the present invention is a light-emitting elementhaving the above-described structure in which the second light-emittinglayer includes n (n is an integer of 2 or larger) layers, and the nlayers contain n kinds of substances having different emission spectraand capable of converting triplet excitation energy into light emission.

Another embodiment of the present invention is a light-emitting elementhaving the above-described structure in which the second light-emittinglayer contains a first phosphorescent substance and a secondphosphorescent substance that have different emission spectra as thesubstance capable of converting triplet excitation energy into lightemission.

Another embodiment of the present invention is a light-emitting elementhaving the above-described structure in which the first phosphorescentsubstance emits light in a red region, the second phosphorescentsubstance emits light in a green region, and the fluorescent substanceemits light in a blue region.

Another embodiment of the present invention is a light-emitting elementhaving the above-described structure in which the first phosphorescentsubstance has a peak of an emission spectrum of 580 nm to 680 nm, thesecond phosphorescent substance has a peak of an emission spectrum of500 nm to 560 nm, and the fluorescent substance has a peak of anemission spectrum of 400 nm to 480 nm.

Another embodiment of the present invention is a light-emitting elementhaving the above-described structure in which the second light-emittinglayer comprises a first phosphorescent layer and a second phosphorescentlayer, the first phosphorescent layer contains the first phosphorescentsubstance, and the second phosphorescent layer contains the secondphosphorescent substance.

Another embodiment of the present invention is a light-emitting elementhaving the above-described structure in which the first phosphorescentsubstance exhibits a carrier-trapping property in the firstphosphorescent layer.

Another embodiment of the present invention is a light-emitting elementhaving the above-described structure in which the carrier-trappingproperty is an electron-trapping property.

Another embodiment of the present invention is a display moduleincluding any of the above-described light-emitting elements.

Another embodiment of the present invention is a lighting moduleincluding any of the above-described light-emitting elements.

Another embodiment of the present invention is a light-emitting deviceincluding any of the above-described light-emitting elements and a unitfor controlling the light-emitting element.

Another embodiment of the present invention is a display deviceincluding any of the above-described light-emitting elements in adisplay portion and a unit for controlling the light-emitting element.

Another embodiment of the present invention is a lighting deviceincluding any of the above-described light-emitting elements in alighting portion and a unit for controlling the light-emitting element.

Another embodiment of the present invention is an electronic applianceincluding any of the above-described light-emitting elements.

Note that the light-emitting device in this specification includes, inits category, an image display device that uses a light-emittingelement. The category of the light-emitting device in this specificationincludes a module in which a light-emitting element is provided with aconnector such as an anisotropic conductive film or a tape carrierpackage (TCP); a module having a TCP at the tip of which a printedwiring board is provided; and a module in which an integrated circuit(IC) is directly mounted on a light-emitting element by a chip on glass(COG) method. Furthermore, the category includes a light-emitting devicethat is used in lighting equipment or the like.

In one embodiment of the present invention, a novel light-emittingelement can be provided.

In one embodiment of the present invention, a multicolor light-emittingelement that utilizes fluorescence and phosphorescence, has a relativelysmall number of layers to be formed, and is advantageous for practicalapplication can be provided.

In another embodiment of the present invention, a multicolorlight-emitting element that utilizes fluorescence and phosphorescenceand has high emission efficiency can be provided.

In another embodiment of the present invention, a multicolorlight-emitting element that utilizes fluorescence and phosphorescence,has a relatively small number of layers to be formed, is advantageousfor practical application, and has high emission efficiency can beprovided.

In another embodiment of the present invention, a display module, alighting module, a light-emitting device, a display device, anelectronic appliance, and a lighting device that can be fabricated atlow cost by using any of the above-described light-emitting elements canbe provided.

In another embodiment of the present invention, a display module, alighting module, a light-emitting device, a display device, anelectronic appliance, and a lighting device that have reduced powerconsumption by using any of the above-described light-emitting elementscan be provided. Note that the description of these effects does notdisturb the existence of other effects. One embodiment of the presentinvention does not necessarily achieve all these effects. Other effectswill be apparent from and can be derived from the description of thespecification, the drawings, the claims, and the like.

BRIEF DESCRIPTION OF DRAWINGS

FIGS. 1A and 1B are conceptual diagrams of light-emitting elements.

FIGS. 2A and 2B are conceptual diagrams of an active matrixlight-emitting device.

FIGS. 3A and 3B are conceptual diagrams of active matrix light-emittingdevices.

FIG. 4 is a conceptual diagram of an active matrix light-emittingdevice.

FIGS. 5A and 5B are conceptual diagrams of a passive matrixlight-emitting device.

FIGS. 6A and 6B illustrate a lighting device.

FIGS. 7A to 7D illustrate electronic appliances.

FIG. 8 illustrates a light source device.

FIG. 9 illustrates a lighting device.

FIG. 10 illustrates a lighting device.

FIG. 11 illustrates in-vehicle display devices and lighting devices.

FIGS. 12A to 12C illustrate an electronic appliance.

FIG. 13 shows the current density-luminance characteristics ofLight-emitting elements 1 to 4.

FIG. 14 shows the luminance-current efficiency characteristics ofLight-emitting elements 1 to 4.

FIG. 15 shows the voltage-luminance characteristics of Light-emittingelements 1 to 4.

FIG. 16 shows the luminance-external quantum efficiency characteristicsof Light-emitting elements 1 to 4.

FIG. 17 shows the emission spectra of Light-emitting elements 1 to 4.

FIG. 18 shows the current density-luminance characteristics ofLight-emitting element 5.

FIG. 19 shows the luminance-current efficiency characteristics ofLight-emitting element 5.

FIG. 20 shows the voltage-luminance characteristics of Light-emittingelement 5.

FIG. 21 shows the luminance-external quantum efficiency characteristicsof Light-emitting element 5.

FIG. 22 shows the emission spectrum of Light-emitting element 5.

FIG. 23 shows the luminance-CIE chromaticity characteristics ofLight-emitting element 5.

FIG. 24 shows the current density-luminance characteristics ofLight-emitting elements 6 and 7.

FIG. 25 shows the luminance-current efficiency characteristics ofLight-emitting elements 6 and 7.

FIG. 26 shows the voltage-luminance characteristics of Light-emittingelements 6 and 7.

FIG. 27 shows the luminance-external quantum efficiency characteristicsof Light-emitting elements 6 and 7.

FIG. 28 shows the emission spectra of Light-emitting elements 6 and 7.

FIG. 29 shows the emission spectrum of Light-emitting element 8.

FIG. 30 shows the current density-luminance characteristics ofLight-emitting element 9.

FIG. 31 shows the luminance-current efficiency characteristics ofLight-emitting element 9.

FIG. 32 shows the voltage-luminance characteristics of Light-emittingelement 9.

FIG. 33 shows the luminance-external quantum efficiency characteristicsof Light-emitting element 9.

FIG. 34 shows the emission spectrum of Light-emitting element 9.

FIG. 35 shows the luminance-power efficiency characteristics of alighting device in Example 5.

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, embodiments of the present invention will be described withreference to the drawings. Note that the present invention is notlimited to the description below, and it is easily understood by thoseskilled in the art that various changes and modifications can be madewithout departing from the spirit and scope of the present invention.Therefore, the invention should not be construed as being limited to thedescription in the following embodiments.

FIG. 1A is a diagram illustrating a light-emitting element of oneembodiment of the present invention. The light-emitting element includesat least a pair of electrodes (a first electrode 101 and a secondelectrode 102) and the EL layer 103 including a light-emitting layer113. The light-emitting layer 113 has a stacked structure in which thefirst light-emitting layer 113 a, the separation layer 113 b, and thesecond light-emitting layer 113 c are stacked in this order to be incontact with one another.

FIG. 1A also illustrates a hole-injection layer 111, a hole-transportlayer 112, an electron-transport layer 114, and an electron-injectionlayer 115 in the EL layer 103. However, this stacked-layer structure isan example, and the structure of the EL layer 103 in the light-emittingelement of one embodiment of the present invention is not limitedthereto. Note that in FIG. 1A, the first electrode 101 functions as ananode, and the second electrode 102 functions as a cathode.

The first light-emitting layer 113 a contains a fluorescent substanceand a host material. The second light-emitting layer 113 c contains afirst organic compound, a second organic compound, and a phosphorescentcompound. In the light-emitting layer having the structure, acombination of the first organic compound and the second organiccompound preferably forms a first exciplex.

This structure enables light originating from the fluorescent substanceto be emitted efficiently from the first light-emitting layer 113 a andlight originating from the phosphorescent substance to be emittedefficiently from the second light-emitting layer 113 c. Note that evenwhen the light-emitting element does not include a charge-generationlayer between the first light-emitting layer 113 a and the secondlight-emitting layer 113 c (i.e., even when the light-emitting elementis not a tandem element), both fluorescence and phosphorescence can beobtained efficiently.

In general, when a fluorescent layer and a phosphorescent layer areincluded in the same EL layer without being separated by acharge-generation layer to emit light, emission efficiency issignificantly reduced. A factor of this is as follows: the tripletexcitation energy of a host material of the fluorescent layer is lowbecause a substance having a condensed aromatic ring (especially, acondensed aromatic hydrocarbon ring) skeleton, which is typified byanthracene, is generally used as the host material, and tripletexcitation energy generated in the phosphorescent layer is transferredto the fluorescent layer, which results in non-radiative decay. Atpresent, it is difficult to obtain a desired emission wavelength,favorable element characteristics, or high reliability without using asubstance having a condensed aromatic ring skeleton for the fluorescentlayer; thus, it is difficult to achieve favorable characteristics of alight-emitting element having the structure in which the fluorescentlayer and the phosphorescent layer are included in the same EL layer.

Since a triplet excited state has a long relaxation time, the diffusionlength of an exciton is long, many of the excitons generated in thephosphorescent layer are transferred to the fluorescent layer because ofdiffusion, and non-radiative decay of the excitons is caused. This makesthe problem more serious.

At present, it is difficult to obtain a desired emission wavelength,favorable element characteristics, or high reliability with the use of amaterial that does not have a condensed aromatic ring skeleton.Therefore, it is difficult to achieve favorable characteristics of alight-emitting element in which a fluorescent layer and a phosphorescentlayer are formed adjacent to each other.

In a light-emitting element of this embodiment, the first organiccompound and the second organic compound form an exciplex in the secondlight-emitting layer 113 c, and the triplet excitation energy istransferred from the exciplex to the phosphorescent substance, so thatlight emission can be obtained. This structure can solve theabove-described problems.

An exciplex is an excited state formed from two kinds of substances (thefirst organic compound and the second organic compound in one embodimentof the present invention). When an exciplex releases energy, the twokinds of substances that have formed the exciplex serve as the originaldifferent substances. In other words, an exciplex itself does not have aground state, and energy transfer between exciplexes or energy transferto an exciplex from another substance is unlikely to occur in principle.

A process in which one of the first organic compound and the secondorganic compound as a cation and the other of the first organic compoundand the second organic compound as an anion are adjacent to each otherto form an exciplex (an electroplex process) is considered dominant forthe generation of the exciplex in the light-emitting element. Even whenone of the first organic compound and the second organic compound comesinto an excited state, the one quickly interacts with the other of thefirst organic compound and the second organic compound, so that to forman exciplex; thus, most excitons in the second light-emitting layer 113c exist as exciplexes. The exciplex has a smaller band gap than thefirst organic compound and the second organic compound. Furthermore,when the first organic compound and the second organic compound areselected such that the exciplex has lower triplet excitation energy thanat least one of (preferably each of) the first organic compound and thesecond organic compound has, energy transfer from the exciplex to thefirst organic compound and the second organic compound hardly occurs. Inaddition, energy transfer between exciplexes hardly occurs as describedabove. As a result, excitation energy of the exciplex is transferred tothe phosphorescent substance and converted into light emission.Accordingly, diffusion of excitons in the second light-emitting layer113 c hardly occurs. Therefore, the above-mentioned problems can besolved.

Here, in the case where the first light-emitting layer 113 a that is afluorescent layer and the second light-emitting layer 113 c that is aphosphorescent layer are in contact with each other, energy transferfrom an exciplex or a phosphorescent dopant to the host material of thefirst light-emitting layer 113 a (especially triplet-triplet energytransfer) slightly occurs at this interface. As described above,excitons of exciplexes are unlikely to diffuse and easily transferred tothe phosphorescent dopant; therefore, an influence of the excitons isrelatively small. However, when the phosphorescent dopant in contactwith the host material of the first light-emitting layer 113 a exists atthe interface, the host material drastically quenches light emission ofthe phosphorescent dopant due to an energy transfer by Dexter mechanism.Accordingly, by providing the separation layer 113 b between the firstlight-emitting layer 113 a and the second light-emitting layer 113 c,energy transfer at the interface between the first light-emitting layer113 a and the second light-emitting layer 113 c can be suppressed, andboth phosphorescence and fluorescence with better characteristics can beemitted.

In one embodiment of the present invention, when the firstlight-emitting layer 113 a has a structure in which a singlet excitedstate is generated easily by triplet-triplet annihilation (T-Tannihilation: TTA), the triplet excitation energy generated in the firstlight-emitting layer 113 a can be converted into fluorescence in thefirst light-emitting layer 113 a. This enables energy loss of thelight-emitting element of one embodiment of the present to be reduced.In order that the light-emitting layer 113 a can have the structure inwhich the single excited state is generated easily by TTA, it ispreferable to select a host material and a fluorescent substance in thefirst light-emitting layer 113 a so that the singlet excitation level ofthe host material is higher than the singlet excitation level of thefluorescent substance and the triplet excitation level of the hostmaterial is lower than the triplet excitation level of the fluorescentsubstance. As a combination of the host material and the fluorescentsubstance that are in such a relation, a combination of a materialhaving an anthracene skeleton as the host material and a material havinga pyrene skeleton as the fluorescent substance, or the like ispreferable.

Note that when the first light-emitting layer 113 a is too thick,emission from the second light-emitting layer 113 c is difficult toobtain. In addition, when the first light-emitting layer 113 a is toothin, emission from the first light-emitting layer 113 a is difficult toobtain. For those reasons, the thickness of the first light-emittinglayer 113 a is preferably greater than or equal to 5 nm and less than orequal to 20 nm.

In the case where the first light-emitting layer 113 a is formed on theanode side, the first light-emitting layer 113 a preferably has ahole-transport property. In that case, a bipolar material having a highhole-transport property is preferably used. A material having ananthracene skeleton is preferable as such a material. Furthermore, whenthe fluorescent substance has a high hole-trapping property (e.g., acondensed aromatic amine compound described later), the concentration ofthe fluorescent substance is preferably lower than or equal to 5%,further preferably higher than or equal to 1% and lower than or equal to4%, still further preferably higher than or equal to 1% and lower thanor equal to 3%, in which case phosphorescence and fluorescence can beobtained in a balanced manner and with high efficiency. Note that thefluorescent substance exhibits a hole-trapping property when the HOMOlevel of the fluorescent substance is higher than the HOMO level of thehost material.

Although there is no limitation on the combination of the first organiccompound and the second organic compound in the second light-emittinglayer 113 c as long as an exciplex can be formed, one organic compoundis preferably a substance having a hole-transport property and the otherorganic compound is preferably a substance having an electron-transportproperty. In that case, a donor-acceptor excited state is formed easily,which allows an exciplex to be formed efficiently. In the case where thecombination of the first organic compound and the second organiccompound is a combination of the substance having a hole-transportproperty and the substance having an electron-transport property, thecarrier balance can be controlled easily by adjusting the mixing ratio.Specifically, the weight ratio of the substance having a hole-transportproperty to the substance having an electron-transport property ispreferably 1:9 to 9:1. Since the carrier balance can be easilycontrolled in the light-emitting element having the above-describedstructure, a recombination region can also be easily adjusted. Thelight-emitting element of one embodiment of the present invention alsohas a feature in that an emission color can be adjusted by controllingthe carrier balance as described above.

The lowest-energy absorption band of the phosphorescent substanceoverlaps the emission spectrum of the first exciplex in the secondlight-emitting layer 113 c, whereby energy transfer from the firstexciplex to the phosphorescent substance is optimized and thelight-emitting element can have favorable emission efficiency. Thedifference in equivalent energy value between a peak wavelength in thelowest-energy absorption band of the phosphorescent substance and a peakwavelength of the emission spectrum of the exciplex is preferably lessthan or equal to 0.2 eV, in which case the overlap between theabsorption band and the emission spectrum is large. Note that thelowest-energy absorption band of the phosphorescent substance ispreferably a triplet absorption band, and in the case where a thermallyactivated delayed fluorescence (TADF) material is used instead of thephosphorescent substance, the lowest-energy absorption band ispreferably a singlet absorption band.

In the light-emitting element of one embodiment of the presentinvention, a light-emitting substance contained in the secondlight-emitting layer 113 c is preferably a substance capable ofconverting triplet excitation energy into light emission. In thisspecification, the term “phosphorescent substance” can be replaced withthe term “TADF material”, and the term “phosphorescent layer” can bereplaced with the term “TADF light-emitting layer”. The TADF material isa substance that can up-convert a triplet excited state into a singletexcited state (i.e., reverse intersystem crossing is possible) using alittle thermal energy and efficiently exhibits light emission(fluorescence) from the singlet excited state. The TADF is efficientlyobtained under the condition where the difference in energy between thetriplet excitation level and the singlet excitation level is greaterthan or equal to 0 eV and less than or equal to 0.2 eV, preferablygreater than or equal to 0 eV and less than or equal to 0.1 eV. Thephosphorescent substance and the TADF material are both substances thatcan convert triplet excitation energy into light emission.

In the light-emitting element of this embodiment, it is preferable thata carrier recombination region be not locally formed but distributed tosome extent. For that, it is preferable that each light-emitting layerhave a moderate degree of carrier-trapping property. In the structurewhere the first light-emitting layer 113 a is formed on the anode sideand the second light-emitting layer 113 c is formed on the cathode side,the fluorescent substance in the first light-emitting layer 113 apreferably has a hole-trapping property, and the phosphorescentsubstance in the second light-emitting layer 113 c preferably has anelectron-trapping property. In the structure where the firstlight-emitting layer 113 a is formed on the cathode side and the secondlight-emitting layer 113 c is formed on the anode side, the fluorescentsubstance in the first light-emitting layer 113 a preferably has anelectron-trapping property, and the phosphorescent substance in thesecond light-emitting layer 113 c preferably has a hole-trappingproperty. Examples of a substance having a high electron-trappingproperty include transition metal complexes (e.g., an iridium complexand a platinum complex) whose ligands include a diazine skeleton such asa pyrimidine skeleton or a pyrazine skeleton. Note that thephosphorescent substance exhibits an electron-trapping property when theLUMO level of the phosphorescent substance is lower than the LUMO levelsof both of the first organic compound and the second organic compound.

Although the separation layer 113 b may be formed with a singlesubstance, the separation layer 113 b preferably contains a substancehaving a hole-transport property and a substance having anelectron-transport property. It is more preferable that these substancesform an exciplex. By changing the mixture ratio of the substance havinga hole-transport property to the substance having an electron-transportproperty, the carrier balance can be easily controlled and lightemission color can be adjusted as in the case of the secondlight-emitting layer 113 c.

It is preferable that the singlet excitation energy and the tripletexcitation energy of a material that forms the separation layer 113 b bethe same as or higher than those of the host material of the firstlight-emitting layer 113 a. Note that in the case where a secondexciplex is formed in the separation layer 113 b, the singlet excitationenergy and the triplet excitation energy of the second exciplex may belower than those of the host material because energy transfer to theexciplex hardly occurs as described above.

The singlet excitation energy and the triplet excitation energy of amaterial that forms the separation layer 113 b are not limited by thesinglet excitation energy and the triplet excitation energy of the firstexciplex in the second light-emitting layer 113 c. In other words, thesinglet excitation energy and the triplet excitation energy of thematerial that forms the separation layer 113 b may be higher or lowerthan those of the first exciplex in the second light-emitting layer 113c. In a general structure, when the excitation energy of the separationlayer is lower than the excitation energy of the second light-emittinglayer 113 c, light emission of the second light-emitting layer 113 c issignificantly reduced; however, in the structure of one embodiment ofthe present invention, most of all excitons in the second light-emittinglayer 113 c exist as exciplexes, so that the excitons hardly diffuse andenergy loss is small.

In the case where the separation layer 113 b contains the substancehaving a hole-transport property and a substance having anelectron-transport property, the combination of these substances ispreferably the same as the combination of the first organic compound andthe second organic compound which form the second light-emitting layer113 c, in which case an increase in drive voltage is suppressed. Thatis, it is preferable that one of the first organic compound and thesecond organic compound be the substance having a hole-transportproperty in the separation layer 113 b and the other of the firstorganic compound and the second organic compound be the substance havingan electron-transport property in the separation layer 113 b. In otherwords, the second exciplex formed in the separation layer 113 b ispreferably the same as the first exciplex formed in the secondlight-emitting layer 113 c.

Note that in the light-emitting element, light emitted from the firstlight-emitting layer 113 a preferably has a peak on the shorterwavelength side than light emitted from the second light-emitting layer113 c. The luminance of a light-emitting element using thephosphorescent substance emitting light with a short wavelength tends todegrade quickly. In view of the above, the fluorescence substanceemitting light with a short wavelength is used, so that a light-emittingelement with less degradation of luminance can be provided. In thislight-emitting element, only the separation layer 113 b with a thicknessof several nanometers is provided between the first light-emitting layer113 a that is a fluorescent layer and the second light-emitting layer113 c that is a phosphorescent layer. Therefore, the number andthickness of layers forming the EL layer in this light-emitting elementare smaller than those in a tandem element; thus, the light-emittingelement of one embodiment of the present invention is cost-effective andsuitable for mass production. In addition, the number of layers formingthe EL layer is small as described above; thus, the thickness of the ELlayer can be small and the light-emitting element is opticallyadvantageous (i.e., the outcoupling efficiency is high). Furthermore,the light-emitting element can have low drive voltage and provide bothfluorescence and phosphorescence efficiently at a drive voltage of 5 Vor lower.

Moreover, although the fluorescent layer and the phosphorescent layerare adjacent to each other, deactivation of the triplet excitationenergy is unlikely to occur owing to the use of the above-describedexciplex in the phosphorescent layer; thus, both phosphorescence andfluorescence can be obtained easily.

In the light-emitting element of this embodiment, light with differentemission wavelengths are obtained from the first light-emitting layer113 a and the second light-emitting layer 113 c, so that thelight-emitting element can be a multicolor light-emitting element.Therefore, the light-emitting element can provide various emissioncolors with a combination of light emitted from a plurality oflight-emitting substances.

Such a light-emitting element is suitable for obtaining white lightemission. When the first light-emitting layer 113 a and the secondlight-emitting layer 113 c emit light of complementary colors, whitelight emission can be obtained. In addition, white light emission with ahigh color rendering property that is formed of three primary colors orfour or more colors can be obtained by using a plurality oflight-emitting substances emitting light with different wavelengths forone or both of the light-emitting layers. In that case, each of thelight-emitting layers may be divided into layers and the divided layersmay contain different light-emitting substances. Such a whitelight-emitting element utilizes phosphorescence, has high emissionefficiency, and can be provided at lower cost because the whitelight-emitting element has a smaller number of layers and a smallerthickness than a tandem light-emitting element. In addition, the whitelight-emitting element improves light extraction efficiency due to itssmall thickness.

Next, an example of the structure of the above-mentioned light-emittingelement is described in detail below with reference to FIG. 1A

A light-emitting element in this embodiment includes, between a pair ofelectrodes, an EL layer including a plurality of layers. In thisembodiment, the light-emitting element includes the first electrode 101,the second electrode 102, and the EL layer 103 provided between thefirst electrode 101 and the second electrode 102. Note that in thisembodiment, the first electrode 101 functions as an anode and the secondelectrode 102 functions as a cathode. Note that the stacking order maybe reversed. In other words, the first light-emitting layer 113 a may beformed on the cathode side and the second light-emitting layer 113 c maybe formed on the anode side.

Since the first electrode 101 functions as the anode, the firstelectrode 101 is preferably formed using any of metals, alloys,electrically conductive compounds with a high work function(specifically, a work function of 4.0 eV or more), mixtures thereof, andthe like. Specific examples include indium oxide-tin oxide (ITO: indiumtin oxide), indium oxide-tin oxide containing silicon or silicon oxide,indium oxide-zinc oxide, and indium oxide containing tungsten oxide andzinc oxide (IWZO). Films of these electrically conductive metal oxidesare usually formed by a sputtering method but may be formed byapplication of a sol-gel method or the like. For example, indiumoxide-zinc oxide is deposited by a sputtering method using a targetobtained by adding 1 wt % to 20 wt % of zinc oxide to indium oxide. Afilm of indium oxide containing tungsten oxide and zinc oxide (IWZO) canbe formed by a sputtering method using a target in which tungsten oxideand zinc oxide are added to indium oxide at 0.5 wt % to 5 wt % and 0.1wt % to 1 wt %, respectively. Besides, gold (Au), platinum (Pt), nickel(Ni), tungsten (W), chromium (Cr), molybdenum (Mo), iron (Fe), cobalt(Co), copper (Cu), palladium (Pd), nitrides of metal materials (e.g.,titanium nitride), and the like can be given. Graphene can also be used.Note that when a composite material described later is used for a layerthat is in contact with the first electrode 101 in the EL layer 103, anelectrode material can be selected regardless of its work function.

There is no particular limitation on the stacked-layer structure of theEL layer 103 as long as the light-emitting layer 113 has theabove-described structure. For example, the EL layer 103 can be formedby combining a hole-injection layer, a hole-transport layer, thelight-emitting layer, an electron-transport layer, an electron-injectionlayer, a carrier-blocking layer, an intermediate layer, and the like asappropriate. In this embodiment, the EL layer 103 has a structure inwhich the hole-injection layer 111, the hole-transport layer 112, thelight-emitting layer 113, the electron-transport layer 114, and theelectron-injection layer 115 are stacked in this order over the firstelectrode 101. Specific examples of materials used for each layer aregiven below.

The hole-injection layer 111 is a layer containing a substance having ahole-injection property. Molybdenum oxide, vanadium oxide, rutheniumoxide, tungsten oxide, manganese oxide, or the like can be used.Alternatively, the hole-injection layer 111 can be formed using aphthalocyanine-based compound such as phthalocyanine (abbreviation:H₂Pc) or copper phthalocyanine (abbreviation: CuPc); an aromatic aminecompound such as4,4′-bis[N-(4-diphenylaminophenyl)-N-phenylamino]biphenyl (abbreviation:DPAB) orN,N′-bis{4-[bis(3-methylphenyl)amino]phenyl}-N,N′-diphenyl-(1,1′-biphenyl)-4,4′-diamine(abbreviation: DNTPD); a high molecule compound such aspoly(3,4-ethylenedioxythiophene)/poly(styrenesulfonic acid) (PEDOT/PSS);or the like.

Alternatively, a composite material in which a substance having ahole-transport property contains a substance having an acceptor propertycan be used for the hole-injection layer 111. Note that the use of sucha substance having a hole-transport property which contains a substancehaving an acceptor property enables selection of a material used to forman electrode regardless of its work function. In other words, besides amaterial having a high work function, a material having a low workfunction can be used for the first electrode 101. As the acceptorsubstance, 7,7,8,8-tetracyano-2,3,5,6-tetrafluoroquinodimethane(abbreviation: F₄-TCNQ), chloranil, and the like can be given. Moreover,oxides of metals belonging to Groups 4 to 8 of the periodic table can beused. Specifically, vanadium oxide, niobium oxide, tantalum oxide,chromium oxide, molybdenum oxide, tungsten oxide, manganese oxide, andrhenium oxide are preferable because of their high electron-acceptingproperties. Among these, molybdenum oxide is especially preferablebecause it is stable in the air, has a low hygroscopic property, and iseasily handled.

As the substance having a hole-transport property used for the compositematerial, any of a variety of organic compounds such as aromatic aminecompounds, carbazole derivatives, aromatic hydrocarbons, and highmolecular compounds (e.g., oligomers, dendrimers, or polymers) can beused. Note that the organic compound used for the composite material ispreferably a substance having a hole-transport property. Specifically, asubstance having a hole mobility of 10⁻⁶ cm²/Vs or higher is preferablyused. Specific examples of the organic compound that can be used as asubstance having a hole-transport property in the composite material aregiven below.

Examples of the aromatic amine compounds areN,N′-di(p-tolyl)-N,N′-diphenyl-p-phenylenediamine (abbreviation:DTDPPA), 4,4′-bis[N-(4-diphenylaminophenyl)-N-phenylamino]biphenyl(abbreviation: DPAB),N,N′-bis{4-[bis(3-methylphenyl)amino]phenyl}-N,N′-diphenyl-(1,1′-biphenyl)-4,4′-diamine(abbreviation: DNTPD), and1,3,5-tris[N-(4-diphenylaminophenyl)-N-phenylamino]benzene(abbreviation: DPA3B).

Specific examples of the carbazole derivatives that can be used for thecomposite material are3-[N-(9-phenylcarbazol-3-yl)-N-phenylamino]-9-phenylcarbazole(abbreviation: PCzPCA1),3,6-bis[N-(9-phenylcarbazol-3-yl)-N-phenylamino]-9-phenylcarbazole(abbreviation: PCzPCA2), and3-[N-(1-naphthyl)-N-(9-phenylcarbazol-3-yl)amino]-9-phenylcarbazole(abbreviation: PCzPCN1).

Other examples of the carbazole derivatives that can be used for thecomposite material are 4,4′-di(N-carbazolyl)biphenyl (abbreviation:CBP), 1,3,5-tris[4-(N-carbazolyl)phenyl]benzene (abbreviation: TCPB),9-[4-(10-phenyl-9-anthryl)phenyl]-9H-carbazole (abbreviation: CzPA), and1,4-bis[4-(N-carbazolyl)phenyl]-2,3,5,6-tetraphenylbenzene.

Examples of the aromatic hydrocarbon that can be used for the compositematerial are 2-tert-butyl-9,10-di(2-naphthyl)anthracene (abbreviation:t-BuDNA), 2-tert-butyl-9,10-di(1-naphthyl)anthracene,9,10-bis(3,5-diphenylphenyl)anthracene (abbreviation: DPPA),2-tert-butyl-9,10-bis(4-phenylphenyl)anthracene (abbreviation: t-BuDBA),9,10-di(2-naphthyl)anthracene (abbreviation: DNA),9,10-diphenylanthracene (abbreviation: DPAnth), 2-tert-butylanthracene(abbreviation: t-BuAnth), 9,10-bis(4-methyl-1-naphthyl)anthracene(abbreviation: DMNA), 2-tert-butyl-9,10-bis[2-(1-naphthyl)phenyl]anthracene,9,10-bis[2-(1-naphthyl)phenyl]anthracene,2,3,6,7-tetramethyl-9,10-di(1-naphthyl)anthracene,2,3,6,7-tetramethyl-9,10-di(2-naphthyl)anthracene, 9,9′-bianthryl,10,10′-diphenyl-9,9′-bianthryl,10,10′-bis(2-phenylphenyl)-9,9′-bianthryl,10,10′-bis[(2,3,4,5,6-pentaphenyl)phenyl]-9,9′-bianthryl, anthracene,tetracene, rubrene, perylene, and 2,5,8,11-tetra(tert-butyl)perylene.

Other examples are pentacene and coronene. The aromatic hydrocarbon thathas a hole mobility of 1×10⁻⁶ cm²/Vs or higher and has 14 to 42 carbonatoms is particularly preferable.

Note that the aromatic hydrocarbons that can be used for the compositematerial may have a vinyl skeleton. Examples of the aromatic hydrocarbonhaving a vinyl group include 4,4′-bis(2,2-diphenylvinyl)biphenyl(abbreviation: DPVBi) and9,10-bis[4-(2,2-diphenylvinyl)phenyl]anthracene (abbreviation: DPVPA).

Other examples are high molecular compounds such aspoly(N-vinylcarbazole) (abbreviation: PVK), poly(4-vinyltriphenylamine)(abbreviation: PVTPA),poly[N-(4-{N′-[4-(4-diphenylamino)phenyl]phenyl-N′-phenylamino}phenyl)methacrylamide](abbreviation: PTPDMA), andpoly[N,N′-bis(4-butylphenyl)-N,N′-bis(phenyl)benzidine] (abbreviation:poly-TPD).

By providing a hole-injection layer, a high hole-injection property canbe achieved to allow a light-emitting element to be driven at a lowvoltage.

The hole-transport layer 112 is a layer containing a substance having ahole-transport property. Examples of the substance having ahole-transport property are aromatic amine compounds such as4,4′-bis[N-(1-naphthyl)-N-phenylamino]biphenyl (abbreviation: NPB),N,N′-bis(3-methylphenyl)-N,N′-diphenyl-[1,1′-biphenyl]-4,4′-diamine(abbreviation: TPD), 4,4′,4″-tris(N,N-diphenylamino)triphenylamine(abbreviation: TDATA),4,4′,4″-tris[N-(3-methylphenyl)-N-phenylamino]triphenylamine(abbreviation: MTDATA),4,4′-bis[N-(spiro-9,9′-bifluoren-2-yl)-N-phenylamino]biphenyl(abbreviation: BSPB), and4-phenyl-4′-(9-phenylfluoren-9-yl)triphenylamine (abbreviation: BPAFLP).The substances listed here have high hole-transport properties and aremainly ones that have a hole mobility of 10⁻⁶ cm²/Vs or higher. Anorganic compound given as an example of the substance having ahole-transport property in the composite material described above canalso be used for the hole-transport layer 112. Moreover, a highmolecular compound such as poly(N-vinylcarbazole) (abbreviation: PVK)and poly(4-vinyltriphenylamine) (abbreviation: PVTPA) can also be used.Note that the layer that contains a substance having a hole-transportproperty is not limited to a single layer, and may be a stack of two ormore layers including any of the above substances.

In the case where the first light-emitting layer 113 a is provided onthe anode side in the light-emitting element of one embodiment of thepresent invention, the HOMO level of a substance used for thehole-transport layer 112 and the HOMO level of a host material in thefirst light-emitting layer 113 a are preferably close to each other (anenergy difference of 0.2 eV or less). This can prevent capture of toomany holes by trap states and enables holes to flow into the separationlayer 113 b and the second light-emitting layer 113 c. Thus,fluorescence and phosphorescence can be easily obtained in a balancedmanner with high efficiency.

The light-emitting layer 113 has the above-mentioned structure. In otherwords, the first light-emitting layer 113 a, the separation layer 113 b,and the second light-emitting layer 113 c are stacked in this order overthe first electrode. A host material and a fluorescent substance arecontained in the first light-emitting layer 113 a. A first organiccompound, a second organic compound, and a substance that can converttriplet excitation energy into luminescence (a phosphorescent compoundor a TADF material) are contained in the second light-emitting layer 113c.

Examples of a material that can be used as the fluorescent substance inthe first light-emitting layer 113 a are given below. Fluorescentmaterials other than those given below can also be used.

Examples of the fluorescent substance are5,6-bis[4-(10-phenyl-9-anthryl)phenyl]-2,2′-bipyridine (abbreviation:PAP2BPy), 5,6-bis[4′-(10-phenyl-9-anthryl)biphenyl-4-yl]-2,2′-bipyridine(abbreviation: PAPP2BPy),N,N′-bis[4-(9-phenyl-9H-fluoren-9-yl)phenyl]-N,N′-diphenyl-pyrene-1,6-diamine(abbreviation: 1,6FLPAPrn),N,N′-bis(3-methylphenyl)-N,N′-bis[3-(9-phenyl-9H-fluoren-9-yl)phenyl]-pyrene-1,6-diamine(abbreviation: 1,6mMemFLPAPrn),N,N′-bis[4-(9H-carbazol-9-yl)phenyl]-N,N′-diphenylstilbene-4,4′-diamine(abbreviation: YGA2S),4-(9H-carbazol-9-yl)-4′-(10-phenyl-9-anthryl)triphenylamine(abbreviation: YGAPA),4-(9H-carbazol-9-yl)-4′-(9,10-diphenyl-2-anthryl)triphenylamine(abbreviation: 2YGAPPA),N,9-diphenyl-N-[4-(10-phenyl-9-anthryl)phenyl]-9H-carbazol-3-amine(abbreviation: PCAPA), perylene, 2,5,8,11-tetra-tert-butylperylene(abbreviation: TBP),4-(10-phenyl-9-anthryl)-4′-(9-phenyl-9H-carbazol-3-yl)triphenylamine(abbreviation: PCBAPA),N,N″-(2-tert-butylanthracene-9,10-diyldi-4,1-phenylene)bis[N,N′,N′-triphenyl-1,4-phenylenediamine](abbreviation: DPABPA),N,9-diphenyl-N-[4-(9,10-diphenyl-2-anthryl)phenyl]-9H-carbazol-3-amine(abbreviation: 2PCAPPA),N-[4-(9,10-diphenyl-2-anthryl)phenyl]-N,N′,N′-triphenyl-1,4-phenylenediamine(abbreviation: 2DPAPPA),N,N,N′,N′,N″,N″,N′″,N′″-octaphenyldibenzo[g,p]chrysene-2,7,10,15-tetraamine(abbreviation: DBC1), coumarin 30,N-(9,10-diphenyl-2-anthryl)-N,9-diphenyl-9H-carbazol-3-amine(abbreviation: 2PCAPA),N-[9,10-bis(1,1′-biphenyl-2-yl)-2-anthryl]-N,9-diphenyl-9H-carbazol-3-amine(abbreviation: 2PCABPhA),N-(9,10-diphenyl-2-anthryl)-N,N′,N′-triphenyl-1,4-phenylenediamine(abbreviation: 2DPAPA),N-[9,10-bis(1,1′-biphenyl-2-yl)-2-anthryl]-N,N′,N′-triphenyl-1,4-phenylenediamine(abbreviation: 2DPABPhA),9,10-bis(1,1′-biphenyl-2-yl)-N-[4-(9H-carbazol-9-yl)phenyl]-N-phenylanthracen-2-amine(abbreviation: 2YGABPhA), N,N,9-triphenylanthracen-9-amine(abbreviation: DPhAPhA), coumarin 545T, N,N′-diphenylquinacridone(abbreviation: DPQd), rubrene,5,12-bis(1,1′-biphenyl-4-yl)-6,11-diphenyltetracene (abbreviation: BPT),2-(2-{2-[4-(dimethylamino)phenyl]ethenyl}-6-methyl-4H-pyran-4-ylidene)propanedinitrile(abbreviation: DCM1),2-{2-methyl-6-[2-(2,3,6,7-tetrahydro-1H,5H-benzo[ij]quinolizin-9-yl)ethenyl]-4H-pyran-4-ylidene}propanedinitrile(abbreviation: DCM2),N,N,N′,N′-tetrakis(4-methylphenyl)tetracene-5,11-diamine (abbreviation:p-mPhTD),7,14-diphenyl-N,N,N′,N′-tetrakis(4-methylphenyl)acenaphtho[1,2-a]fluoranthene-3,10-diamine(abbreviation: p-mPhAFD),2-{2-isopropyl-6-[2-(1,1,7,7-tetramethyl-2,3,6,7-tetrahydro-1H,5H-benzo[ij]quinolizin-9-yl)ethenyl]-4H-pyran-4-ylidene}propanedinitrile(abbreviation: DCJTI),2-{2-tert-butyl-6-[2-(1,1,7,7-tetramethyl-2,3,6,7-tetrahydro-1H,5H-benzo[ij]quinolizin-9-yl)ethenyl]-4H-pyran-4-ylidene}propanedinitrile(abbreviation: DCJTB), 2-(2,6-bis{2-[4-(dimethylamino)phenyl]ethenyl}-4H-pyran-4-ylidene)propanedinitrile(abbreviation: BisDCM), and2-{2,6-bis[2-(8-methoxy-1,1,7,7-tetramethyl-2,3,6,7-tetrahydro-1H,5H-benzoquinolizin-9-yl)ethenyl]-4H-pyran-4-ylidene}propanedinitrile(abbreviation: BisDCJTM). Condensed aromatic diamine compounds typifiedby pyrenediamine compounds such as 1,6FLPAPrn and 1,6mMemFLPAPm areparticularly preferable because of their high hole-trapping properties,high emission efficiency, and high reliability.

Examples of a substance that can be used as the host material in thefirst light-emitting layer 113 a are given below.

The examples include anthracene compounds such as9-phenyl-3-[4-(10-phenyl-9-anthryl)phenyl]-9H-carbazole (abbreviation:PCzPA), 3-[4-(1-naphthyl)-phenyl]-9-phenyl-9H-carbazole (abbreviation:PCPN), 9-[4-(10-phenyl-9-anthryl)phenyl]-9H-carbazole (abbreviation:CzPA), 7-[4-(10-phenyl-9-anthryl)phenyl]-7H-dibenzo[c,g]carbazole(abbreviation: cgDBCzPA),6-[3-(9,10-diphenyl-2-anthryl)phenyl]-benzo[b]naphtho[1,2-d]furan(abbreviation: 2mBnfPPA), and9-phenyl-10-{4-(9-phenyl-9H-fluoren-9-yl)biphenyl-4′-yl}anthracene(abbreviation: FLPPA). The use of a substance having an anthraceneskeleton as the host material enables a light-emitting layer that hashigh emission efficiency and durability to be provided. In particular,CzPA, cgDBCzPA, 2mBnfPPA, and PCzPA are preferable because of theirexcellent characteristics.

A phosphorescent substance and a TADF material can be used as thesubstance that can convert triplet excitation energy into luminescencein the second light-emitting layer 113 c. Examples of the phosphorescentsubstance and the TADF material are given below.

Examples of the phosphorescent substance are an organometallic iridiumcomplex having a 4H-triazole skeleton, such astris{2-[5-(2-methylphenyl)-4-(2,6-dimethylphenyl)-4H-1,2,4-triazol-3-yl-kN2]phenyl-kC}iridium(III)(abbreviation: Ir(mpptz-dmp)₃),tris(5-methyl-3,4-diphenyl-4H-1,2,4-triazolato)iridium(III)(abbreviation: Ir(Mptz)₃), ortris[4-(3-biphenyl)-5-isopropyl-3-phenyl-4H-1,2,4-triazolato]iridium(III)(abbreviation: Ir(iPrptz-3b)₃); an organometallic iridium complex havinga 1H-triazole skeleton, such astris[3-methyl-1-(2-methylphenyl)-5-phenyl-1H-1,2,4-triazolato]iridium(III)(abbreviation: Ir(Mptz1-mp)₃) ortris(1-methyl-5-phenyl-3-propyl-1H-1,2,4-triazolato)iridium(III)(abbreviation: Ir(Prptz1-Me)₃); an organometallic iridium complex havingan imidazole skeleton, such asfac-tris[1-(2,6-diisopropylphenyl)-2-phenyl-1H-imidazole]iridium(III)(abbreviation: Ir(iPrpmi)₃), ortris[3-(2,6-dimethylphenyl)-7-methylimidazo[1,2-f]phenanthridinato]iridium(III)(abbreviation: Ir(dmpimpt-Me)₃); and an organometallic iridium complexin which a phenylpyridine derivative having an electron-withdrawinggroup is a ligand, such asbis[2-(4′,6′-difluorophenyl)pyridinato-N,C^(2′)]iridium(III)tetrakis(1-pyrazolyl)borate (abbreviation: FIr6),bis[2-(4′,6′-difluorophenyl)pyridinato-N,C]iridium(III) picolinate(abbreviation: FIrpic), bis{2-[3′,5′-bis(trifluoromethyl)phenyl]pyridinato-N,C^(2′)}iridium(III)picolinate (abbreviation: Ir(CF₃ppy)₂(pic)), orbis[2-(4′,6′-difluorophenyl)pyridinato-N,C²]iridium(III) acetylacetonate(abbreviation: FIr(acac)). These are compounds emitting bluephosphorescence and have an emission peak at 440 nm to 520 nm.

Other examples are organometallic iridium complexes having pyrimidineskeletons, such as tris(4-methyl-6-phenylpyrimidinato)iridium(III)(abbreviation: Ir(mppm)₃),tris(4-t-butyl-6-phenylpyrimidinato)iridium(III) (abbreviation:Ir(tBuppm)₃),(acetylacetonato)bis(6-methyl-4-phenylpyrimidinato)iridium(III)(abbreviation: Ir(mppm)₂(acac)),bis[2-(6-tert-butyl-4-pyrimidinyl-κN3)phenyl-kC](2,4-pentanedionato-κ²O,O′)iridium(III)(abbreviation: Ir(tBuppm)₂(acac)),(acetylacetonato)bis[4-(2-norbornyl)-6-phenylpyrimidinato]iridium(III)(endo- and exo-mixture) (abbreviation: Ir(nbppm)₂(acac)),(acetylacetonato)bis[5-methyl-6-(2-methylphenyl)-4-phenylpyrimidinato]iridium(III)(abbreviation: Ir(mpmppm)₂(acac)), and(acetylacetonato)bis(4,6-diphenylpyrimidinato)iridium(III)(abbreviation: Ir(dppm)₂(acac)); organometallic iridium complexes havingpyrazine skeletons, such as(acetylacetonato)bis(3,5-dimethyl-2-phenylpyrazinato)iridium(III)(abbreviation: Ir(mppr-Me)₂(acac)) and(acetylacetonato)bis(5-isopropyl-3-methyl-2-phenylpyrazinato)iridium(III)(abbreviation: Ir(mppr-iPr)₂(acac)); organometallic iridium complexeshaving pyridine skeletons, such astris(2-phenylpyridinato-N,C^(2′))iridium(III) (abbreviation: Ir(ppy)₃),bis(2-phenylpyridinato-N,C^(2′))iridium(III) acetylacetonate(abbreviation: Ir(ppy)₂acac)), bis(benzo[h]quinolinato)iridium(III)acetylacetonate (abbreviation: Ir(bzq)₂(acac)),tris(benzo[h]quinolinato)iridium(III) (abbreviation: Ir(bzq)₃),tris(2-phenylquinolinato-N,C²′)iridium(III) (abbreviation: Ir(pq)₃), andbis(2-phenylquinolinato-N,C^(2′))iridium(III) acetylacetonate(abbreviation: Ir(pq)₂(acac)); and a rare earth metal complex such astris(acetylacetonato) (monophenanthroline)terbium(III) (abbreviation:Tb(acac)₃(Phen)).

These are mainly compounds emitting green phosphorescence and have anemission peak at 500 nm to 600 nm. Note that an organometallic iridiumcomplex having a pyrimidine skeleton has distinctively high reliabilityand emission efficiency and is thus especially preferable.

Other examples are(diisobutyrylmethanato)bis[4,6-bis(3-methylphenyl)pyrimidinato]iridium(III)(abbreviation: Ir(5mdppm)₂(dibm)),bis[4,6-bis(3-methylphenyl)pyrimidinato](dipivaloylmethanato)iridium(III)(abbreviation: Ir(5mdppm)₂(dpm)), andbis[4,6-di(naphthalen-1-yl)pyrimidinato](dipivaloylmethanato)iridium(III)(abbreviation: Ir(d1npm)₂(dpm)); organometallic iridium complexes havingpyrazine skeletons, such as(acetylacetonato)bis(2,3,5-triphenylpyrazinato)iridium(III)(abbreviation: Ir(tppr)₂(acac)),bis(2,3,5-triphenylpyrazinato)(dipivaloylmethanato)iridium(III)(abbreviation: Ir(tppr)₂(dpm)), or(acetylacetonato)bis[2,3-bis(4-fluorophenyl)quinoxalinato]iridium(III)(abbreviation: Ir(Fdpq)₂(acac)); organometallic iridium complexes havingpyridine skeletons, such astris(1-phenylisoquinolinato-N,C^(2′))iridium(III) (abbreviation:Ir(piq)₃) andbis(1-phenylisoquinolinato-N,C^(2′))iridium(III)acetylacetonate(abbreviation: Ir(piq)₂(acac)); a platinum complex such as2,3,7,8,12,13,17,18-octaethyl-21H,23H-porphyrin platinum(II)(abbreviation: PtOEP); and rare earth metal complexes such astris(1,3-diphenyl-1,3-propanedionato) (monophenanthroline)europium(III)(abbreviation: Eu(DBM)₃(Phen)) andtris[1-(2-thenoyl)-3,3,3-trifluoroacetonato](monophenanthroline)europium(III) (abbreviation: Eu(TTA)₃(Phen)). These are compounds emitting redphosphorescence and have an emission peak at 600 nm to 700 nm. Theorganometallic iridium complex having a pyrazine skeleton can providered light emission with favorable chromaticity.

Phosphorescent materials other than those given above may be used.

Materials given below can be used as the TADF material.

A fullerene, a derivative thereof, an acridine derivative such asproflavine, eosin, or the like can be used. A metal-containing porphyrinsuch as a porphyrin containing magnesium (Mg), zinc (Zn), cadmium (Cd),tin (Sn), platinum (Pt), indium (In), or palladium (Pd) can be used.Examples of the metal-containing porphyrin are a protoporphyrin-tinfluoride complex (SnF₂(Proto IX)), a mesoporphyrin-tin fluoride complex(SnF₂(Meso IX)), a hematoporphyrin-tin fluoride complex (SnF₂(HematoIX)), a coproporphyrin tetramethyl ester-tin fluoride complex(SnF₂(Copro III-4Me)), an octaethylporphyrin-tin fluoride complex(SnF₂(OEP)), an etioporphyrin-tin fluoride complex (SnF₂(Etio I)), andan octaethylporphyrin-platinum chloride complex (PtCl₂(OEP)), which arerepresented by Structural Formulae below.

Alternatively, a heterocyclic compound including a π-electron richheteroaromatic ring and a π-electron deficient heteroaromatic ring canbe used, such as2-(biphenyl-4-yl)-4,6-bis(12-phenylindolo[2,3-a]carbazol-11-yl)-1,3,5-triazine(abbreviation: PIC-TRZ), which is represented by Structural Formulabelow. The heterocyclic compound is preferably used because of theπ-electron rich heteroaromatic ring and the π-electron deficientheteroaromatic ring, for which the electron-transport property and thehole-transport property are high. Note that a substance in which theπ-electron rich heteroaromatic ring is directly bonded to the π-electrondeficient heteroaromatic ring is particularly preferably used becausethe donor property of the π-electron rich heteroaromatic ring and theacceptor property of the π-electron deficient heteroaromatic ring areboth increased and the energy difference between the S₁ level and the T₁level becomes small.

There is no particular limitation on the materials that can be used asthe first organic compound and the second organic compound as long asthe combination of the materials satisfies the above-mentionedconditions. A variety of carrier-transport materials can be selected.

Examples of the substance having an electron-transport property are aheterocyclic compound having a polyazole skeleton, such asbis(10-hydroxybenzo[h]quinolinato)beryllium(II) (abbreviation: BeBq₂),bis(2-methyl-8-quinolinolato)(4-phenylphenolato)aluminum(III)(abbreviation: BAlq), bis(8-quinolinolato)zinc(II) (abbreviation: Znq),bis[2-(2-benzoxazolyl)phenolato]zinc(II) (abbreviation: ZnPBO), orbis[2-(2-benzothiazolyl)phenolato]zinc(II) (abbreviation: ZnBTZ); aheterocyclic compound having a polyazole skeleton such as2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole (abbreviation:PBD), 3-(4-biphenylyl)-4-phenyl-5-(4-tert-butylphenyl)-1,2,4-triazole(abbreviation: TAZ),1,3-bis[5-(p-tert-butylphenyl)-1,3,4-oxadiazol-2-yl]benzene(abbreviation: OXD-7),9-[4-(5-phenyl-1,3,4-oxadiazol-2-yl)phenyl]-9H-carbazole (abbreviation:CO11), 2,2′,2″-(1,3,5-benzenetriyl)tris(1-phenyl-1H-benzimidazole)(abbreviation: TPBI), or2-[3-(dibenzothiophen-4-yl)phenyl]-1-phenyl-1H-benzimidazole(abbreviation: mDBTBIm-II); a heterocyclic compound having a diazineskeleton, such as2-[3-(dibenzothiophen-4-yl)phenyl]dibenzo[f,h]quinoxaline (abbreviation:2mDBTPDBq-II),2-[3′-(dibenzothiophen-4-yl)biphenyl-3-yl]dibenzo[f,h]quinoxaline(abbreviation: 2mDBTBPDBq-II),2-[3′-(9H-carbazol-9-yl)biphenyl-3-yl]dibenzo[f,h]quinoxaline(abbreviation: 2mCzBPDBq), 4,6-bis[3-(phenanthren-9-yl)phenyl]pyrimidine(abbreviation: 4,6mPnP2Pm), or4,6-bis[3-(4-dibenzothienyl)phenyl]pyrimidine (abbreviation:4,6mDBTP2Pm-II); and a heterocyclic compound having a pyridine skeleton,such as 2-[3′-(dibenzothiophen-4-yl)biphenyl-3-yl]dibenzo[f,h]quinoline(abbreviation: 2mDBTBPDBQu-II),3,5-bis[3-(9H-carbazol-9-yl)phenyl]pyridine (abbreviation: 35DCzPPy), or1,3,5-tri[3-(3-pyridyl)phenyl]benzene (abbreviation: TmPyPB). Among theabove materials, a heterocyclic compound having a diazine skeleton and aheterocyclic compound having a pyridine skeleton have high reliabilityand are thus preferable. Specifically, a heterocyclic compound having adiazine (pyrimidine or pyrazine) skeleton has a high electron-transportproperty to contribute to a reduction in drive voltage.

Examples of the substance having a hole-transport property are acompound having an aromatic amine skeleton, such as4,4′-bis[N-(1-naphthyl)-N-phenylamino]biphenyl (abbreviation: NPB),N,N′-bis(3-methylphenyl)-N,N′-diphenyl-[1,1′-biphenyl]-4,4′-diamine(abbreviation: TPD),4,4′-bis[N-(spiro-9,9′-bifluoren-2-yl)-N-phenylamino]biphenyl(abbreviation: BSPB), 4-phenyl-4′-(9-phenylfluoren-9-yl)triphenylamine(abbreviation: BPAFLP), 4-phenyl-3′-(9-phenylfluoren-9-yl)triphenylamine(abbreviation: mBPAFLP),4-phenyl-4′-(9-phenyl-9H-carbazol-3-yl)triphenylamine (abbreviation:PCBA1BP), 4,4′-diphenyl-4″-(9-phenyl-9H-carbazol-3-yl)triphenylamine(abbreviation: PCBBi1BP),4-(1-naphthyl)-4′-(9-phenyl-9H-carbazol-3-yl)triphenylamine(abbreviation: PCBANB),4,4′-di(1-naphthyl)-4″-(9-phenyl-9H-carbazol-3-yl)triphenylamine(abbreviation: PCBNBB),9,9-dimethyl-N-phenyl-N-[4-(9-phenyl-9H-carbazol-3-yl)phenyl]fluoren-2-amine(abbreviation: PCBAF), orN-phenyl-N-[4-(9-phenyl-9H-carbazol-3-yl)phenyl]spiro-9,9′-bifluoren-2-amine(abbreviation: PCBASF); a compound having a carbazole skeleton, such as1,3-bis(N-carbazolyl)benzene (abbreviation: mCP),4,4′-di(N-carbazolyl)biphenyl (abbreviation: CBP),3,6-bis(3,5-diphenylphenyl)-9-phenylcarbazole (abbreviation: CzTP), or3,3′-bis(9-phenyl-9H-carbazole) (abbreviation: PCCP); a compound havinga thiophene skeleton such as4,4′,4″-(benzene-1,3,5-triyl)tri(dibenzothiophene) (abbreviation:DBT3P-II), 2,8-diphenyl-4-[4-(9-phenyl-9H-fluoren-9-yl)phenyl]dibenzothiophene(abbreviation: DBTFLP-III), or4-[4-(9-phenyl-9H-fluoren-9-yl)phenyl]-6-phenyldibenzothiophene(abbreviation: DBTFLP-IV); and a compound having a furan skeleton, suchas 4,4′,4″-(benzene-1,3,5-triyl)tri(dibenzofuran) (abbreviation:DBF3P-II) or4-{3-[3-(9-phenyl-9H-fluoren-9-yl)phenyl]phenyl}dibenzofuran(abbreviation: mmDBFFLBi-II). Among the above materials, a compoundhaving an aromatic amine skeleton and a compound having a carbazoleskeleton are preferable because these compounds are highly reliable andhave high hole-transport properties to contribute to a reduction indrive voltage.

Carrier-transport materials can be selected from a variety of substancesas well as from the carrier-transport materials given above. Note thatas the first organic compound and the second organic compound,substances having a triplet level (a difference in energy between aground state and a triplet excited state) higher than the triplet levelof the phosphorescent compound are preferably selected. In addition, itis preferable that the combination of the first organic compound and thesecond organic compound be selected so that an exciplex which exhibitslight emission whose wavelength overlaps a wavelength of a lowest-energyabsorption band of the phosphorescent substance is formed.

Furthermore, the combination of a substance having an electron-transportproperty as one of the first organic compound and the second organiccompound and a substance having a hole-transport property as the otherorganic compound is advantageous for the formation of an exciplex. Thetransport property of the light-emitting layer can be easily adjustedand a recombination region can be easily adjusted by changing thecontained amount of each compound. The ratio of the contained amount ofthe substance having a hole-transport property to the contained amountof the substance having an electron-transport property may be 1:9 to9:1.

As materials that form the separation layer 113 b, the above-mentionedmaterials that can be used as the first organic compound and the secondorganic compound can be used.

The second light-emitting layer 113 c may be divided into two or morelayers, and the divided layers preferably contain differentlight-emitting substances. In particular, a structure in which thesecond light-emitting layer 113 c is divided into a first phosphorescentlayer that emits red light (i.e., light having an emission spectrum peakat 580 nm to 680 nm) and a second phosphorescent layer that emits greenlight (i.e., light having an emission spectrum peak at 500 nm to 560 nm)and the first light-emitting layer 113 a emits blue light (i.e., lighthaving an emission spectrum peak at 400 nm to 480 nm) is preferablyemployed, in which case white light emission with a favorable colorrendering property can be obtained. Note that in that case, the firstlight-emitting layer 113 a, the first phosphorescent layer, and thesecond phosphorescent layer are preferably stacked in this order forhigh durability. Furthermore the first light-emitting layer 113 a ispreferably formed on the anode side, in which case favorablecharacteristics can be obtained.

The electron-transport layer 114 is a layer containing a substancehaving an electron-transport property. For example, theelectron-transport layer 114 is formed using a metal complex having aquinoline skeleton or a benzoquinoline skeleton, such astris(8-quinolinolato)aluminum (abbreviation: Alq),tris(4-methyl-8-quinolinolato)aluminum (abbreviation: Almq₃),bis(10-hydroxybenzo[h]quinolinato)beryllium (abbreviation: BeBq₂), orbis(2-methyl-8-quinolinolato)(4-phenylphenolato)aluminum (abbreviation:BAlq), or the like. A metal complex having an oxazole-based orthiazole-based ligand, such as bis[2-(2-hydroxyphenyl)benzoxazolato]zinc(abbreviation: Zn(BOX)₂) or bis[2-(2-hydroxyphenyl)benzothiazolato]zinc(abbreviation: Zn(BTZ)₂), or the like can also be used. Besides themetal complexes,2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole (abbreviation:PBD), 1,3-bis[5-(p-tert-butylphenyl)-1,3,4-oxadiazol-2-yl]benzene(abbreviation: OXD-7),3-(4-biphenylyl)-4-phenyl-5-(4-tert-butylphenyl)-1,2,4-triazole(abbreviation: TAZ), bathophenanthroline (abbreviation: BPhen),bathocuproine (abbreviation: BCP), or the like can also be used. Thesubstances listed here have high electron-transport properties and aremainly ones that have an electron mobility of 10⁻⁶ cm²/Vs or higher.Note that any of the substances having electron-transport properties,which are listed above, may be used for the electron-transport layer114.

The electron-transport layer 114 is not limited to a single layer, andmay be a stack of two or more different layers each containing any ofthe substances listed above

A layer for controlling transport of electron carriers may be providedbetween the electron-transport layer and the light-emitting layer. Thisis a layer formed by addition of a small amount of a substance having ahigh electron-trapping property to the aforementioned materials having ahigh electron-transport property, and the layer is capable of adjustingcarrier balance by retarding transport of electron carriers. Such astructure is very effective in preventing a problem (e.g., a reductionin element lifetime) caused when electrons pass through thelight-emitting layer.

An electron-injection layer 115 may be provided in contact with thesecond electrode 102 between the electron-transport layer 114 and thesecond electrode 102. For the electron-injection layer 115, an alkalimetal, an alkaline earth metal, or a compound thereof, such as lithiumfluoride (LiF), cesium fluoride (CsF), or calcium fluoride (CaF₂), canbe used. For example, a layer that is formed using a substance having anelectron-transport property and contains an alkali metal, an alkalineearth metal, or a compound thereof can be used. Note that a layer thatis formed using a substance having an electron-transport property andcontains an alkali metal or an alkaline earth metal is preferably usedas the electron-injection layer 115, in which case electron injectionfrom the second electrode 102 is efficiently performed.

For the second electrode 102, any of metals, alloys, electricallyconductive compounds, and mixtures thereof which have a low workfunction (specifically, a work function of 3.8 eV or less) or the likecan be used. Specific examples of such a cathode material are elementsbelonging to Groups 1 and 2 of the periodic table, such as alkali metals(e.g., lithium (Li) and cesium (Cs)), magnesium (Mg), calcium (Ca), andstrontium (Sr), alloys thereof (e.g., MgAg and AlLi), rare earth metalssuch as europium (Eu) and ytterbium (Yb), and alloys thereof. However,when the electron-injection layer is provided between the secondelectrode 102 and the electron-transport layer, for the second electrode102, any of a variety of conductive materials such as Al, Ag, ITO, orindium oxide-tin oxide containing silicon or silicon oxide can be usedregardless of the work function. These conductive materials can bedeposited by a sputtering method, an ink-jet method, a spin coatingmethod, or the like.

Any of a variety of methods can be used to form the EL layer 103regardless whether it is a dry process or a wet process. For example, avacuum evaporation method, an ink jet method, or a spin coating methodmay be employed. A different formation method may be employed for eachelectrode or each layer.

The electrode may be formed by a wet method using a sol-gel method, orby a wet method using paste of a metal material. Alternatively, theelectrode may be formed by a dry method such as a sputtering method or avacuum evaporation method.

Light emission is extracted out through one or both of the firstelectrode 101 and the second electrode 102. Therefore, one or both ofthe first electrode 101 and the second electrode 102 arelight-transmitting electrodes. In the case where only the firstelectrode 101 is a light-transmitting electrode, light emission isextracted through the first electrode 101. In contrast, in the casewhere only the second electrode 102 is a light-transmitting electrode,light emission is extracted through the second electrode 102. In thecase where both the first electrode 101 and the second electrode 102 arelight-transmitting electrodes, light emission is extracted through thefirst electrode 101 and the second electrode 102.

Note that the structure of the layer provided between the firstelectrode 101 and the second electrode 102 is not limited to theabove-described structure. Preferably, a light-emitting region whereholes and electrons recombine is positioned away from the firstelectrode 101 and the second electrode 102 so that quenching due to theproximity of the light-emitting region and a metal used for electrodesand carrier-injection layers can be prevented.

In order that transfer of energy from an exciton generated in thelight-emitting layer can be suppressed, preferably, the hole-transportlayer and the electron-transport layer that are in contact with thelight-emitting layer 113, particularly a carrier-transport layer incontact with a side closer to the light-emitting region in thelight-emitting layer 113, are formed using a substance having a widerband gap than the light-emitting substance of the light-emitting layeror the emission center substance contained in the light-emitting layer.

The light-emitting element of this embodiment may be formed over a glasssubstrate, a quartz substrate, a semiconductor substrate, a plasticsubstrate (polyester, polyolefin, polyamide (e.g., nylon or aramid),polyimide, polycarbonate, or an acrylic resin). Alternatively, thelight-emitting element may be formed over a glass substrate, a quartzsubstrate, or a semiconductor substrate and then transferred to aplastic substrate.

In a light-emitting device, although one light-emitting element may befabricated over one substrate, a plurality of light-emitting elementsmay be fabricated over one substrate. With a plurality of light-emittingelements as described above formed over one substrate, a lighting devicein which elements are separated or a passive-matrix light-emittingdevice can be fabricated. A light-emitting element may be formed over anelectrode electrically connected to a field-effect transistor (FET), forexample, that is formed over a substrate of glass, plastic, or the like,so that an active matrix light-emitting device in which the FET controlsthe drive of the light-emitting element can be fabricated. Note that thestructure of the FET is not particularly limited. In addition,crystallinity of a semiconductor used for the FET is not particularlylimited either; an amorphous semiconductor or a crystallinesemiconductor may be used. In addition, a driver circuit formed in anFET substrate may be formed with an n-type FET and a p-type FET, or witheither an n-type FET or a p-type FET.

Note that this embodiment can be combined with any of the otherembodiments as appropriate.

Next, one mode a light-emitting element with a structure in which aplurality of light-emitting units are stacked (hereinafter also referredto as a stacked-type element) is described with reference to FIG. 1B. Inthis light-emitting element, a plurality of light-emitting units areprovided between a first electrode and a second electrode. Onelight-emitting unit has a structure similar to that of the EL layer 103,which is illustrated in FIG. 1A. In other words, the light-emittingelement illustrated in FIG. 1A includes a single light-emitting unit;the light-emitting element in this embodiment includes a plurality oflight-emitting units.

In FIG. 1B, a first light-emitting unit 511 and a second light-emittingunit 512 are stacked between a first electrode 501 and a secondelectrode 502, and a charge-generation layer 513 is provided between thefirst light-emitting unit 511 and the second light-emitting unit 512.The first electrode 501 and the second electrode 502 correspond,respectively, to the first electrode 101 and the second electrode 102illustrated in FIG. 1A, and the materials given in the description forFIG. 1A can be used. Furthermore, the first light-emitting unit 511 andthe second light-emitting unit 512 may have the same structure ordifferent structures.

The charge-generation layer 513 contains a composite material of anorganic compound and a metal oxide. As the composite material of theorganic compound and the metal oxide, the composite material which canbe used for the hole-injection layer 111 illustrated in FIG. 1A can beused. Note that when a surface of a light-emitting unit on the anodeside is in contact with a charge generation layer, the charge generationlayer can also serve as a hole-transport layer of the light-emittingunit; thus, a hole-transport layer does not need to be formed in thelight-emitting unit.

The charge generation layer 513 may have a stacked-layer structure of alayer containing the composite material of an organic compound and ametal oxide and a layer containing another material. For example, alayer containing a composite material of the organic compound and themetal oxide may be combined with a layer containing a compound of asubstance selected from substances with an electron-donating propertyand a substance with an electron-transport property. Moreover, a layercontaining a composite material of the organic compound and the metaloxide may be combined with a transparent conductive film.

The light-emitting element having two light-emitting units is describedwith reference to FIG. 1B; however, the present invention can besimilarly applied to a light-emitting element in which three or morelight-emitting units are stacked. With a plurality of light-emittingunits partitioned by the charge generation layer between a pair ofelectrodes as in the light-emitting element of this embodiment, it ispossible to provide a light-emitting element which can emit light withhigh luminance with the current density kept low. As a result, it ispossible to achieve a light-emitting device which can be driven at lowvoltage and has low power consumption.

When the above-described structure of the light-emitting layer 113 isapplied to at least one of the plurality of units, the number ofmanufacturing steps of the unit can be reduced; thus, a multicolorlight-emitting element which is advantageous for practical applicationcan be provided.

The above-described structure can be combined with any of the structuresin this embodiment and the other embodiments.

(Light-Emitting Device)

Next, a light-emitting device of one embodiment of the present inventionis described.

The light-emitting device of one embodiment of the present invention ismanufactured using the above-described light-emitting element. Note thatFIG. 2A is a top view illustrating the light-emitting device and FIG. 2Bis a cross-sectional view taken along the lines A-B and C-D in FIG. 2A.This light-emitting device includes a driver circuit portion (sourceline driver circuit) 601, a pixel portion 602, and a driver circuitportion (gate line driver circuit) 603, which control light emission ofthe light-emitting element and are denoted by dotted lines. A referencenumeral 604 denotes a sealing substrate; 605, a sealant; and 607, aspace surrounded by the sealant 605. The space 607 may be filled with adry inert gas or a resin for solid sealing. The sealant 605 may be thesame as or different from the resin for solid sealing.

Note that a lead wiring 608 is a wiring for transmitting signals to beinput to the source line driver circuit 601 and the gate line drivercircuit 603 and for receiving a video signal, a clock signal, a startsignal, a reset signal, and the like from an FPC (flexible printedcircuit) 609 serving as an external input terminal. Although only theFPC is illustrated here, a printed wiring board (PWB) may be attached tothe FPC. The light-emitting device in the present specificationincludes, in its category, not only the light-emitting device itself butalso the light-emitting device provided with the FPC or the PWB.

Next, a cross-sectional structure is described with reference to FIG.2B. The driver circuit portion and the pixel portion are formed over anelement substrate 610. The source line driver circuit 601, which is adriver circuit portion, and one of the pixels in the pixel portion 602are illustrated here.

In the source line driver circuit 601, a CMOS circuit is formed in whichan n-channel FET 623 and a p-channel FET 624 are combined. In addition,the driver circuit may be formed with any of a variety of circuits suchas a CMOS circuit, a PMOS circuit, and an NMOS circuit. Although adriver-integrated type in which a driver circuit is formed over asubstrate is described in this embodiment, one embodiment of the presentinvention is not limited to this type, and the driver circuit can beformed outside the substrate.

The pixel portion 602 includes a plurality of pixels including aswitching FET 611, a current controlling FET 612, and a first electrode613 electrically connected to a drain of the current controlling FET612. An insulator 614 is formed to cover end portions of the firstelectrode 613. In this embodiment, the insulator 614 is formed using apositive photosensitive acrylic resin film.

In order to improve the coverage, the insulator 614 preferably has acurved surface with curvature at an upper end portion or a lower endportion thereof. For example, in the case where a positivephotosensitive acrylic resin is used for a material of the insulator614, only the upper end portion of the insulator 614 preferably has asurface with a curvature radius (0.2 μm to 3 μm). As the insulator 614,either a negative photosensitive resin or a positive photosensitiveresin can be used.

An EL layer 616 and a second electrode 617 are formed over the firstelectrode 613. As a material used for the first electrode 613functioning as an anode, a material having a high work function ispreferably used. For example, a single-layer film of an ITO film, anindium tin oxide film containing silicon, an indium oxide filmcontaining zinc oxide at 2 wt % to 20 wt %, a titanium nitride film, achromium film, a tungsten film, a Zn film, a Pt film, or the like, astack including a titanium nitride film and a film containing aluminumas its main component, a stack including three layers of a titaniumnitride film, a film containing aluminum as its main component, and atitanium nitride film, or the like can be used. The stacked-layerstructure enables low wiring resistance, favorable ohmic contact, and afunction as an anode.

The EL layer 616 is formed by any of a variety of methods such as anevaporation method using an evaporation mask, an inkjet method, and aspin coating method. The EL layer 616 has a structure similar to thatillustrated in FIG. 1A or FIG. 1B. As another material contained in theEL layer 616, any of low-molecular-weight compounds andhigh-molecular-weight compounds (including oligomers and dendrimers) maybe used.

As a material used for the second electrode 617, which is formed overthe EL layer 616 and functions as a cathode, a material having a lowwork function (e.g., Al, Mg, Li, Ca, or an alloy or a compound thereof,such as MgAg, MgIn, or AlLi) is preferably used. In the case where lightgenerated in the EL layer 616 passes through the second electrode 617, astack of a thin metal film and a transparent conductive film (e.g., ITO,indium oxide containing zinc oxide at 2 wt % to 20 wt %, indium tinoxide containing silicon, or zinc oxide (ZnO)) is preferably used forthe second electrode 617.

The sealing substrate 604 is attached to the element substrate 610 withthe sealant 605, so that the light-emitting element 618 is provided inthe space 607 surrounded by the element substrate 610, the sealingsubstrate 604, and the sealant 605. The space 607 may be filled withfiller, and may be filled with an inert gas (such as nitrogen or argon),a resin, or the sealant 605. It is preferable that the sealing substratebe provided with a recessed portion and a drying agent be provided inthe recessed portion, in which case deterioration due to influence ofmoisture can be suppressed.

An epoxy-based resin or glass frit is preferably used for the sealant605. It is preferable that such a material not transmit moisture oroxygen as much as possible. As the sealing substrate 604, a glasssubstrate, a quartz substrate, or a plastic substrate formed of fiberreinforced plastic (FRP), poly(vinyl fluoride) (PVF), polyester,acrylic, or the like can be used.

The light-emitting device of one embodiment of the present invention canhave small power consumption. The lighting device can also beinexpensive.

FIGS. 3A and 3B each illustrate an example of a light-emitting device inwhich full color display is achieved by forming a light-emitting elementexhibiting white light emission and providing a coloring layer (a colorfilter) and the like. In FIG. 3A, a substrate 1001, a base insulatingfilm 1002, a gate insulating film 1003, gate electrodes 1006, 1007, and1008, a first interlayer insulating film 1020, a second interlayerinsulating film 1021, a peripheral portion 1042, a pixel portion 1040, adriver circuit portion 1041, first electrodes 1024W, 1024R, 1024G, and1024B of light-emitting elements, a partition wall 1025, an EL layer1028, a second electrode 1029 of the light-emitting elements, a sealingsubstrate 1031, a sealant 1032, and the like are illustrated.

In FIG. 3A, coloring layers (a red coloring layer 1034R, a greencoloring layer 1034G, and a blue coloring layer 1034B) are provided on atransparent base material 1033. A black layer (a black matrix) 1035 maybe additionally provided. The transparent base material 1033 providedwith the coloring layers and the black layer is positioned and fixed tothe substrate 1001. Note that the coloring layers and the black layerare covered with an overcoat layer 1036. In FIG. 3A, light emitted frompart of the light-emitting layer does not pass through the coloringlayers, while light emitted from the other part of the light-emittinglayer passes through the coloring layers. Since light that does not passthrough the coloring layers is white and light that passes through anyone of the coloring layers is red, blue, or green, an image can bedisplayed using pixels of the four colors.

FIG. 3B illustrates an example in which the coloring layers (the redcoloring layer 1034R, the green coloring layer 1034G, and the bluecoloring layer 1034B) are provided between the gate insulating film 1003and the first interlayer insulating film 1020. As shown in FIG. 3B, thecoloring layers may be provided between the substrate 1001 and thesealing substrate 1031.

The above-described light-emitting device has a structure in which lightis extracted from the substrate 1001 side where the FETs are formed (abottom emission structure), but may have a structure in which light isextracted from the sealing substrate 1031 side (a top emissionstructure). FIG. 4 is a cross-sectional view of a light-emitting devicehaving a top emission structure. In this case, a substrate that does nottransmit light can be used as the substrate 1001. The process up to thestep of forming of a connection electrode which connects the FET and theanode of the light-emitting element is performed in a manner similar tothat of the light-emitting device having a bottom emission structure.Then, a third interlayer insulating film 1037 is formed to cover anelectrode 1022. This insulating film may have a planarization function.The third interlayer insulating film 1037 can be formed using a materialsimilar to that of the second interlayer insulating film, and canalternatively be formed using any other materials.

The first electrodes 1024W, 1024R, 1024G, and 1024B of thelight-emitting elements each serve as an anode here, but may serve as acathode. In the case of a light-emitting device having a top emissionstructure as illustrated in FIG. 4, the first electrodes are preferablyreflective electrodes. The EL layer 1028 is formed to have a structuresimilar to the structure of the EL layer 103 illustrated in FIG. 1A,with which white light emission can be obtained.

In the case of a top emission structure as illustrated in FIG. 4,sealing can be performed with the sealing substrate 1031 on which thecoloring layers (the red coloring layer 1034R, the green coloring layer1034G, and the blue coloring layer 1034B) are provided. The sealingsubstrate 1031 may be provided with the black layer (the black matrix)1035 that is positioned between pixels. The coloring layers (the redcoloring layer 1034R, the green coloring layer 1034G, and the bluecoloring layer 1034B) and the black layer (the black matrix) 1035 may becovered with an overcoat layer. Note that a light-transmitting substrateis used as the sealing substrate 1031.

Although an example in which full color display is performed using fourcolors of red, green, blue, and white is shown here, there is noparticular limitation and full color display using three colors of red,green, and blue may be performed.

The light-emitting device in this embodiment is fabricated using thelight-emitting element illustrated in FIG. 1A or FIG. 1B and thus canhave favorable characteristics. Specifically, since the light-emittingelement illustrated in FIG. 1A or FIG. 1B has high emission efficiency,the light-emitting device can have reduced power consumption. Inaddition, since the light-emitting element illustrated in FIG. 1A orFIG. 1B is relatively easily mass-produced, the light-emitting devicecan be provided at low cost.

An active matrix light-emitting device is described above, whereas apassive matrix light-emitting device is described below. FIGS. 5A and 5Billustrate a passive matrix light-emitting device fabricated using oneembodiment of the present invention. FIG. 5A is a perspective view ofthe light-emitting device, and FIG. 5B is a cross-sectional view takenalong the line X-Y in FIG. 5A. In FIGS. 5A and 5B, an EL layer 955 isprovided between an electrode 952 and an electrode 956 over a substrate951. An end portion of the electrode 952 is covered with an insulatinglayer 953. In addition, a partition layer 954 is provided over theinsulating layer 953. The sidewalls of the partition layer 954 slope sothat the distance between one sidewall and the other sidewall graduallydecreases toward the surface of the substrate. In other words, a crosssection taken along the direction of the short side of the partitionlayer 954 is trapezoidal, and the base (a side which is in the samedirection as a plane direction of the insulating layer 953 and incontact with the insulating layer 953) is shorter than the upper side (aside which is in the same direction as the plane direction of theinsulating layer 953 and not in contact with the insulating layer 953).By providing the partition layer 954 in such a manner, a defect of thelight-emitting element due to static electricity or the like can beprevented. The passive matrix light-emitting device also includes thelight-emitting element illustrated in FIG. 1A or FIG. 1B, which has highemission efficiency, and thus can have less power consumption. Moreover,since the light-emitting element is easily mass-produced, thelight-emitting device can be provided at low cost.

Since many minute light-emitting elements arranged in a matrix in thelight-emitting device described above can each be controlled, thelight-emitting device can be suitably used as a display device fordisplaying images.

(Lighting Device)

An example in which the light-emitting element illustrated in FIG. 1A orFIG. 1B is used for a lighting device is described with reference toFIGS. 6A and 6B. FIG. 6B is a top view of the lighting device, and FIG.6A is a cross-sectional view taken along the line e-f in FIG. 6B.

In the lighting device in this embodiment, a first electrode 401 isformed over a substrate 400 which is a support and has alight-transmitting property. The first electrode 401 corresponds to thefirst electrode 101 in FIG. 1A. When light is extracted through thefirst electrode 401 side, the first electrode 401 is formed using amaterial having a light-transmitting property.

A pad 412 for applying voltage to a second electrode 404 is providedover the substrate 400.

An EL layer 403 is formed over the first electrode 401. The structure ofthe EL layer 403 corresponds to, for example, the structure of the ELlayer 103 in FIG. 1A, or the structure in which the first light-emittingunit 511, the second light-emitting unit 512, and the charge-generationlayer 513 in FIG. 1B are combined. For these structures, thecorresponding description can be referred to.

The second electrode 404 is formed to cover the EL layer 403. The secondelectrode 404 corresponds to the second electrode 102 in FIG. 1A. Thesecond electrode 404 is formed using a material having high reflectancewhen light is extracted through the first electrode 401 side. The secondelectrode 404 is connected to the pad 412, whereby voltage is appliedthereto.

As described above, the lighting device of one embodiment of the presentinvention includes a light-emitting element including the firstelectrode 401, the EL layer 403, and the second electrode 404.

The light-emitting element having the above structure is fixed to asealing substrate 407 with sealants 405 and 406 and sealing isperformed, whereby the lighting device is completed. It is possible touse only either the sealant 405 or the sealant 406. In addition, theinner sealant 406 (not illustrated in FIG. 6B) can be mixed with adesiccant that enables moisture to be adsorbed, which results inimproved reliability.

When parts of the pad 412 and the first electrode 401 are extended tothe outside of the sealants 405 and 406, the extended parts can serve asexternal input terminals. An IC chip 420 mounted with a converter or thelike may be provided over the external input terminals.

As described above, since the lighting device described in thisembodiment includes the light-emitting element illustrated in FIG. 1A orFIG. 1B as an EL element, the lighting device can have low powerconsumption. In addition, the light-emitting device can have low drivevoltage. Furthermore, the light-emitting device can be inexpensive.

(Electronic Appliance)

Next, examples of electronic appliances each including thelight-emitting element illustrated in FIG. 1A or FIG. 1B are described.The light-emitting element illustrated in FIG. 1A or FIG. 1B has highemission efficiency and reduced power consumption. As a result, theelectronic appliances described in this embodiment can each include alight-emitting portion having reduced power consumption. Thelight-emitting element illustrated in FIG. 1A or FIG. 1B includes asmaller number of layers to be formed; thus, the electronic appliancescan be inexpensive.

Examples of the electronic appliance to which the above light-emittingelement is applied include television devices (also referred to as TV ortelevision receivers), monitors for computers and the like, cameras suchas digital cameras and digital video cameras, digital photo frames,mobile phones (also referred to as cellular phones or cellular phonedevices), portable game machines, portable information terminals, audioplayback devices, and large game machines such as pachinko machines.Specific examples of these electronic appliances are given below.

FIG. 7A illustrates an example of a television device. In the televisiondevice, a display portion 7103 is incorporated in a housing 7101. Inaddition, here, the housing 7101 is supported by a stand 7105. Imagescan be displayed on the display portion 7103 where the light-emittingelements illustrated in FIG. 1A or FIG. 1B are arranged in a matrix.

The television device can be operated with an operation switch of thehousing 7101 or a separate remote controller 7110. With operation keys7109 of the remote controller 7110, channels and volume can becontrolled and images displayed on the display portion 7103 can becontrolled. Furthermore, the remote controller 7110 may be provided witha display portion 7107 for displaying data output from the remotecontroller 7110.

Note that the television device is provided with a receiver, a modem,and the like. With the receiver, general television broadcasts can bereceived. Moreover, when the display device is connected to acommunication network with or without wires via the modem, one-way (froma sender to a receiver) or two-way (between a sender and a receiver orbetween receivers) data communication can be performed.

FIG. 7B1 illustrates a computer that includes a main body 7201, ahousing 7202, a display portion 7203, a keyboard 7204, an externalconnection port 7205, a pointing device 7206, and the like. Note thatthis computer is manufactured by using light-emitting elements arrangedin a matrix in the display portion 7203, which are the same as thatillustrated in FIG. 1A or FIG. 1B. The computer illustrated in FIG. 7B 1may have a structure illustrated in FIG. 7B2. A computer illustrated inFIG. 7B2 is provided with a second display portion 7210 instead of thekeyboard 7204 and the pointing device 7206. The second display portion7210 is a touch screen, and input can be performed by operation ofdisplay for input on the second display portion 7210 with a finger or adedicated pen. The second display portion 7210 can also display imagesother than the display for input. The display portion 7203 may also be atouchscreen. Connecting the two screens with a hinge can preventtroubles; for example, the screens can be prevented from being crackedor broken while the computer is being stored or carried. Note that thiscomputer is manufactured by arranging the light-emitting elementsillustrated in FIG. 1A or FIG. 1B in a matrix in the display portion7203.

FIG. 7C illustrates a portable game machine that includes two housings,a housing 7301 and a housing 7302, which are connected with a jointportion 7303 so that the portable game machine can be opened or folded.The housing 7301 incorporates a display portion 7304 including thelight-emitting elements illustrated in FIG. 1A or FIG. 1B and arrangedin a matrix, and the housing 7302 incorporates a display portion 7305.In addition, the portable game machine illustrated in FIG. 7C includes aspeaker portion 7306, a recording medium insertion portion 7307, an LEDlamp 7308, an input means (an operation key 7309, a connection terminal7310, a sensor 7311 (a sensor having a function of measuring force,displacement, position, speed, acceleration, angular velocity,rotational frequency, distance, light, liquid, magnetism, temperature,chemical substance, sound, time, hardness, electric field, current,voltage, electric power, radiation, flow rate, humidity, gradient,oscillation, odor, or infrared rays), or a microphone 7312), and thelike. Needless to say, the structure of the portable game machine is notlimited to the above structure as long as the display portion includingthe light-emitting elements each of which is illustrated in FIG. 1A orFIG. 1B and which are arranged in a matrix is used as at least eitherthe display portion 7304 or the display portion 7305, or both, and thestructure can include other accessories as appropriate. The portablegame machine illustrated in FIG. 7C has a function of reading out aprogram or data stored in a storage medium to display it on the displayportion, and a function of sharing information with another portablegame machine by wireless communication. The portable game machineillustrated in FIG. 7C can have a variety of functions withoutlimitation to the above functions.

FIG. 7D illustrates an example of a mobile phone. The mobile phone isprovided with a display portion 7402 incorporated in a housing 7401, anoperation button 7403, an external connection port 7404, a speaker 7405,a microphone 7406, and the like. Note that the mobile phone has thedisplay portion 7402 including the light-emitting elements each of whichis illustrated in FIG. 1A or FIG. 1B and which are arranged in a matrix.

When the display portion 7402 of the mobile phone illustrated in FIG. 7Dis touched with a finger or the like, data can be input into the mobilephone. In this case, operations such as making a call and creating ane-mail can be performed by touch on the display portion 7402 with afinger or the like.

There are mainly three screen modes of the display portion 7402. Thefirst mode is a display mode mainly for displaying images. The secondmode is an input mode mainly for inputting data such as text. The thirdmode is a display-and-input mode in which two modes of the display modeand the input mode are combined.

For example, in the case of making a call or composing an e-mail, a textinput mode mainly for inputting text is selected for the display portion7402 so that text displayed on a screen can be inputted. In this case,it is preferable to display a keyboard or number buttons on almost theentire screen of the display portion 7402.

When a detection device which includes a sensor for detectinginclination, such as a gyroscope or an acceleration sensor, is providedinside the mobile phone, the direction of the mobile phone (whether themobile phone is placed horizontally or vertically for a landscape modeor a portrait mode) is determined so that display on the screen of thedisplay portion 7402 can be automatically switched.

The screen modes are switched by touch on the display portion 7402 oroperation with the operation button 7403 of the housing 7401. The screenmodes can be switched depending on the kind of images displayed on thedisplay portion 7402. For example, when a signal of an image displayedon the display portion is a signal of moving image data, the screen modeis switched to the display mode. When the signal is a signal of textdata, the screen mode is switched to the input mode.

Moreover, in the input mode, when input by touching the display portion7402 is not performed for a certain period while a signal detected by anoptical sensor in the display portion 7402 is detected, the screen modemay be controlled so as to be switched from the input mode to thedisplay mode.

The display portion 7402 may function as an image sensor. For example,an image of a palm print, a fingerprint, or the like is taken by touchon the display portion 7402 with the palm or the finger, wherebypersonal authentication can be performed. In addition, by providing abacklight or a sensing light source that emits near-infrared light inthe display portion, an image of a finger vein, a palm vein, or the likecan be taken.

As described above, the application range of the light-emitting deviceincluding the light-emitting element illustrated in FIG. 1A or FIG. 1Bis so wide that the light-emitting device can be applied to electronicappliances in a variety of fields. By using the light-emitting elementillustrated in FIG. 1A or FIG. 1B, an electronic appliance havingreduced power consumption can be obtained.

FIG. 8 illustrates an example of a liquid crystal display device usingthe light-emitting element illustrated in FIG. 1A or FIG. 1B for abacklight. The liquid crystal display device illustrated in FIG. 8includes a housing 901, a liquid crystal layer 902, a backlight unit903, and a housing 904. The liquid crystal layer 902 is connected to adriver IC 905. The light-emitting element illustrated in FIG. 1A or FIG.1B is used for the backlight unit 903, to which current is suppliedthrough a terminal 906.

The light-emitting element illustrated in FIG. 1A or FIG. 1B is used forthe backlight of the liquid crystal display device; thus, the backlightcan have reduced power consumption. In addition, the use of thelight-emitting element illustrated in FIG. 1A or FIG. 1B enablesfabrication of a planar-emission lighting device and further alarger-area planar-emission lighting device; therefore, the backlightcan be a larger-area backlight, and the liquid crystal display devicecan also be a larger-area device. Furthermore, the light-emitting deviceusing the light-emitting element illustrated in FIG. 1A or FIG. 1B canbe thinner than a conventional one; accordingly, the display device canalso be thinner.

FIG. 9 illustrates an example in which the light-emitting elementillustrated in FIG. 1A or FIG. 1B is used for a table lamp that is alighting device. The table lamp illustrated in FIG. 9 includes a housing2001 and a light source 2002. The lighting element illustrated in FIG.1A or FIG. 1B are used for the light source 2002.

FIG. 10 illustrates an example in which the light-emitting elementillustrated in FIG. 1A or FIG. 1B is used for an indoor lighting device3001. Since the light-emitting element illustrated in FIG. 1A or FIG. 1Bcan have a large area, the light-emitting element can be used for alarge-area lighting device. Furthermore, since the light-emittingelement illustrated in FIG. 1A or FIG. 1B is thin, the light-emittingelement can be used for a lighting device having a reduced thickness.

The light-emitting element illustrated in FIG. 1A or FIG. 1B can also beused for an automobile windshield or an automobile dashboard. FIG. 11illustrates one mode in which the light-emitting element illustrated inFIG. 1A or FIG. 1B is used for an automobile windshield and anautomobile dashboard. Displays regions 5000 to 5005 each include thelight-emitting element illustrated in FIG. 1A or FIG. 1B.

The display region 5000 and the display region 5001 are provided in theautomobile windshield in which the light-emitting elements illustratedin FIG. 1A or FIG. 1B are incorporated. The light-emitting elementillustrated in FIG. 1A or FIG. 1B can be formed into what is called asee-through display device, through which the opposite side can be seen,by including a first electrode and a second electrode formed ofelectrodes having light-transmitting properties. Such a see-throughdisplay device does not hinder the vision and thus can be provided inthe automobile windshield. Note that in the case where a transistor fordriving or the like is provided, a transistor having alight-transmitting property, such as an organic transistor using anorganic semiconductor material or a transistor using an oxidesemiconductor, is preferably used.

A display region 5002 is provided in a pillar portion in which thelight-emitting element illustrated in FIG. 1A or FIG. 1B areincorporated. The display region 5002 can compensate for the viewhindered by the pillar portion by showing an image taken by an imagingunit provided in the car body. Similarly, the display region 5003provided in the dashboard can compensate for the view hindered by thecar body by showing an image taken by an imaging unit provided in theoutside of the car body, which leads to elimination of blind areas andenhancement of safety. Showing an image so as to compensate for the areathat a driver cannot see, makes it possible for the driver to confirmsafety easily and comfortably.

The display region 5004 and the display region 5005 can provide avariety of kinds of information such as navigation data, a speedometer,a tachometer, a mileage, a fuel meter, a gearshift indicator, andair-condition setting. The contents or layout of the display can bechanged by a user as appropriate. Note that such information can also beshown by the display regions 5000 to 5003. The display regions 5000 to5005 can also be used as lighting devices.

The light-emitting element illustrated in FIG. 1A or FIG. 1B can havehigh emission efficiency and low power consumption. Therefore, load on abattery is small even when a number of large screens such as the displayregions 5000 to 5005 are provided, which provides comfortable use. Forthat reason, the light-emitting device and the lighting device each ofwhich includes the light-emitting element illustrated in FIG. 1A or FIG.1B can be suitably used as an in-vehicle light-emitting device and anin-vehicle lighting device.

FIGS. 12A and 12B illustrate an example of a foldable tablet terminal.In FIG. 12A, the tablet terminal is opened and includes a housing 9630,a display portion 9631 a, a display portion 9631 b, a display-modeswitching button 9034, a power button 9035, a power-saving-modeswitching button 9036, a clip 9033, and an operation button 9038. Notethat in the tablet terminal, one or both of the display portion 9631 aand the display portion 9631 b is/are formed using a light-emittingdevice which includes the light-emitting element illustrated in FIG. 1Aor FIG. 1B.

Part of the display portion 9631 a can be a touchscreen region 9632 aand data can be input when a displayed operation key 9637 is touched.Although a structure in which a half region in the display portion 9631a has only a display function and the other half region also has atouchscreen function is illustrated as an example, the structure of thedisplay portion 9631 a is not limited thereto. The whole area of thedisplay portion 9631 a may have a touchscreen function. For example, thewhole area of the display portion 9631 a can display keyboard buttonsand serve as a touchscreen while the display portion 9631 b can be usedas a display screen.

Like the display portion 9631 a, part of the display portion 9631 b canbe a touchscreen region 9632 b. When a switching button 9639 forshowing/hiding a keyboard on the touchscreen is touched with a finger, astylus, or the like, the keyboard can be displayed on the displayportion 9631 b.

Touch input can be performed concurrently on the touchscreen regions9632 a and 9632 b.

The display-mode switching button 9034 allows switching between aportrait mode and a landscape mode, and between monochrome display andcolor display, for example. With the power-saving-mode switching button9036, the luminance of display can be optimized in accordance with theamount of external light at the time when the tablet terminal is in use,which is detected with an optical sensor incorporated in the tabletterminal. The tablet terminal may include another detection device suchas a sensor for detecting orientation (e.g., a gyroscope or anacceleration sensor) in addition to the optical sensor.

Although the display portion 9631 a and the display portion 9631 b havethe same display area in FIG. 12A, one embodiment of the presentinvention is not limited to this example. The display portion 9631 a andthe display portion 9631 b may have different areas or different displayquality. For example, one of them may be a display panel that candisplay higher-definition images than the other.

In FIG. 12B, the tablet terminal is folded and includes the housing9630, a solar cell 9633, a charge and discharge control circuit 9634, abattery 9635, and a DC-to-DC converter 9636. Note that FIG. 12Billustrates an example in which the charge and discharge control circuit9634 includes the battery 9635 and the DC-to-DC converter 9636.

Since the tablet terminal is foldable, the housing 9630 can be closedwhen the tablet terminal is not used. Thus, the display portions 9631 aand 9631 b can be protected, whereby a tablet terminal with highendurance and high reliability for long-term use can be provided.

The tablet terminal illustrated in FIGS. 12A and 12B can also have afunction of displaying various kinds of data (e.g., a still image, amoving image, and a text image), a function of displaying a calendar, adate, the time, or the like on the display portion, a touch-inputfunction of operating or editing data displayed on the display portionby touch input, a function of controlling processing by various kinds ofsoftware (programs), and the like.

The solar cell 9633, which is attached on the surface of the tabletterminal, supplies electric power to a touchscreen, a display portion,an image signal processor, and the like. Note that the solar cell 9633is preferably provided on one or two surfaces of the housing 9630, inwhich case the battery 9635 can be charged efficiently.

The structure and operation of the charge and discharge control circuit9634 illustrated in FIG. 12B are described with reference to a blockdiagram of FIG. 12C. FIG. 12C illustrates the solar cell 9633, thebattery 9635, the DC-to-DC converter 9636, a converter 9638, switchesSW1 to SW3, and the display portion 9631. The battery 9635, the DC-to-DCconverter 9636, the converter 9638, and the switches SW1 to SW3correspond to the charge and discharge control circuit 9634 in FIG. 12B.

First, an example of operation in the case where power is generated bythe solar cell 9633 using external light is described. The voltage ofpower generated by the solar cell is raised or lowered by the DC-to-DCconverter 9636 so that the power has voltage for charging the battery9635. Then, when power supplied from the battery 9635 charged by thesolar cell 9633 is used for the operation of the display portion 9631,the switch SW1 is turned on and the voltage of the power is raised orlowered by the converter 9638 so as to be voltage needed for the displayportion 9631. In addition, when display on the display portion 9631 isnot performed, the switch SW1 is turned off and a switch SW2 is turnedon so that charge of the battery 9635 may be performed.

Although the solar cell 9633 is described as an example of a powergeneration means, the power generation means is not particularlylimited, and the battery 9635 may be charged by another power generationmeans such as a piezoelectric element or a thermoelectric conversionelement (Peltier element). The battery 9635 may be charged by anon-contact power transmission module that is capable of charging bytransmitting and receiving power by wireless (without contact), oranother charge means used in combination, and the power generation meansis not necessarily provided.

One embodiment of the present invention is not limited to the tabletterminal having the shape illustrated in FIGS. 12A to 12C as long as thedisplay portion 9631 is included.

Example 1

In this example, methods of fabricating Light-emitting elements 1 to 3each of which is one embodiment of the present invention andLight-emitting element 4 which is a comparative light-emitting elementand characteristics thereof are described. Structural formulae oforganic compounds used for Light-emitting elements 1 to 4 are shownbelow.

(Method of Fabricating Light-Emitting Element 1)

A film of indium tin oxide containing silicon oxide (ITSO) was formedover a glass substrate by a sputtering method to form the firstelectrode 101. The thickness was 110 nm and the electrode area was 2mm×2 mm. Here, the first electrode 101 functions as an anode of thelight-emitting element.

Next, in pretreatment for forming the light-emitting element over thesubstrate, a surface of the substrate was washed with water and baked at200° C. for 1 hour, and then UV ozone treatment was performed for 370seconds.

After that, the substrate was transferred into a vacuum evaporationapparatus where the pressure had been reduced to approximately 10⁻⁴ Pa,and was subjected to vacuum baking at 170° C. for 30 minutes in aheating chamber of the vacuum evaporation apparatus, and then thesubstrate was cooled down for approximately 30 minutes.

Then, the substrate provided with the first electrode 101 was fixed to asubstrate holder provided in the vacuum evaporation apparatus so thatthe surface on which the first electrode 101 was formed faced downward.The pressure in the vacuum evaporation apparatus was reduced to about10⁻⁴ Pa. After that, on the first electrode 101,4,4′,4″-(benzene-1,3,5-triyl)tri(dibenzothiophene) (abbreviation:DBT3P-II) represented by Structural Formula (i) and molybdenum(VI) oxidewere deposited by co-evaporation by an evaporation method usingresistance heating to form the hole-injection layer 111. The thicknesswas set to 40 nm, and the weight ratio of DBT3P-II to molybdenum oxidewas adjusted to 4:2 (=DBT3P-II: molybdenum oxide). Note that theco-evaporation method refers to an evaporation method in whichevaporation is carried out from a plurality of evaporation sources atthe same time in one treatment chamber.

Next, on the hole-injection layer 111,3-[4-(9-phenanthryl)-phenyl]-9-phenyl-9H-carbazole (abbreviation: PCPPn)represented by Structural Formula (ii) was deposited to a thickness of20 nm; thus, the hole-transport layer 112 was formed.

On the hole-transport layer 112,7-[4-(10-phenyl-9-anthryl)phenyl]-7H-dibenzo[c,g]carbazole(abbreviation: cgDBCzPA) represented by Structural Formula (iii) andN,N′-bis(3-methylphenyl)-N,N′-bis[3-(9-phenyl-9H-fluoren-9-yl)phenyl]-pyrene-1,6-diamine(abbreviation: 1,6mMemFLPAPrn) represented by Structural Formula (iv)were deposited by co-evaporation to a thickness of 5 nm such that theweight ratio of cgDBCzPA to 1,6mMemFLPAPrn was 1:0.03 (=cgDBCzPA:1,6mMemFLPAPrn); thus, the first light-emitting layer 113 a that was afluorescent layer was formed. Next,2-[3′-(dibenzothiophen-4-yl)biphenyl-3-yl]dibenzo[f,h]quinoxaline(abbreviation: 2mDBTBPDBq-II) represented by Structural Formula (v) andN-(1,1′-biphenyl-4-yl)-N-[4-(9-phenyl-9H-carbazol-3-yl)phenyl]-9,9-dimethyl-9H-fluoren-2-amine(abbreviation: PCBBiF) represented by Structural Formula (vi) weredeposited by co-evaporation to a thickness of 2 nm such that the weightratio of 2mDBTBPDBq-II to PCBBiF was 0.6:0.4 (=2mDBTBPDBq-II:PCBBiF);thus, the separation layer 113 b was formed. Then, the firstphosphorescent layer 113 c-1 was formed in such a manner that2mDBTBPDBq-II, PCBBiF, andbis{4,6-dimethyl-2-[5-(2,6-dimethylphenyl)-3-(3,5-dimethylphenyl)-2-pyrazinyl-κN]phenyl-κC}(2,4-pentanedionato-κ²O,O′)iridium(III)(abbreviation: [Ir(dmdppr-dmp)₂(acac)]) represented by StructuralFormula (vii) were deposited by co-evaporation to a thickness of 5 nmsuch that the weight ratio of 2mDBTBPDBq-II to PCBBiF and[Ir(dmdppr-dmp)₂(acac)] was 0.2:0.8:0.05(=2mDBTBPDBq-II:PCBBiF:[Ir(dmdppr-dmp)₂(acac)]); the secondphosphorescent layer 113 c-2 was successively formed in such a mannerthat 2mDBTBPDBq-II, PCBBiF, andbis[2-(6-tert-butyl-4-pyrimidinyl-κN3)phenyl-κC](2,4-pentanedionato-κ²O,O)iridium(III)(abbreviation: [Ir(tBuppm)₂(acac)]) represented by Structural Formula(viii) were deposited by co-evaporation to a thickness of 20 nm suchthat the weight ratio of 2mDBTBPDBq-II to PCBBiF and [Ir(tBuppm)₂(acac)]was 0.7:0.3:0.05 (=2mDBTBPDBq-II:PCBBiF:[Ir(tBuppm)₂(acac)]). Throughthe above steps, the second light-emitting layer 113 c that was aphosphorescent layer was formed. In Light-emitting element 1, thelight-emitting layer 113 includes the first light-emitting layer 113 aand the second light-emitting layer 113 c.

Note that 2mDBTBPDBq-II and PCBBiF form an exciplex in thephosphorescent layer (the second light-emitting layer 113 c). Thisemission wavelength overlaps absorption bands on the longest wavelengthsides of [Ir(dmdppr-dmp)₂(acac)] and [Ir(tBuppm)₂(acac)], so that energytransfer efficiency is high.

The singlet excitation energy of cgDBCzPA that is a host material in thefluorescent layer (the first light-emitting layer 113 a) is higher thanthe singlet excitation energy of 1,6mMemFLPAPrn that is a fluorescentsubstance. In addition, the triplet excitation energy of cgDBCzPA islower than the triplet excitation energy of 1,6mMemFLPAPm. Thus, in thefluorescent layer (the first light-emitting layer 113 a), regenerationof a singlet exciton associated with triplet-triplet annihilation andlight emission are obtained easily.

After that, on the phosphorescent layer (the second light-emitting layer113 c), 2mDBTBPDBq-II was deposited to a thickness of 10 nm, andbathophenanthroline (abbreviation: BPhen) represented by StructuralFormula (ix) was deposited to a thickness of 15 nm to form theelectron-transport layer 114.

After the formation of the electron-transport layer 114, lithiumfluoride (LiF) was deposited by evaporation to a thickness of 1 nm toform the electron-injection layer 115. Finally, aluminum was depositedby evaporation to a thickness of 200 nm to form the second electrode 102functioning as a cathode. Through the above-described steps,Light-emitting element 1 of this example was fabricated.

Note that in all the above evaporation steps, evaporation was performedby a resistance-heating method.

(Method of Fabricating Light-Emitting Elements 2 and 3)

Light-emitting element 2 and Light-emitting element 3 are elements inwhich the thickness of the separation layer 113 b of Light-emittingelement 1 is changed to 5 nm and 10 nm, respectively.

(Method of Fabricating Light-Emitting Element 4)

Light-emitting element 4 has a structure in which the separation layer113 b is removed from the structure of Light-emitting element 1.

In a glove box under a nitrogen atmosphere, each of Light-emittingelements 1 to 4 was sealed with a glass substrate so as not to beexposed to the air (specifically, a sealant was applied onto an outeredge of the element, and at the time of sealing, UV treatment wasperformed and then heat treatment was performed at 80° C. for 1 hour).Then, the reliability of these light-emitting elements was measured.Note that the measurement was performed at room temperature (in theatmosphere kept at 25° C.).

Table 1 shows the element structures of Light-emitting elements 1 to 4.

TABLE 1 Hole- Hole- First light- Electron- Electron- injection transportemitting Separation Second light- transport injection layer layer layerlayer emitting layer layer layer DBT3P- PCPPn cgDBCzPA: * ** *** 2mDBTBPhen LiF II:MoOx 20 nm 1,6mMem X 5 nm 20 nm BPDBq-II 15 nm 1 nm 2:1FLPAPm 10 nm 40 nm 1:0.03 5 nm * 2mDBTBPDBq-II:PCBBiF = 0.6:0.4 X:Light-emitting element 1 2 nm, Light-emitting element 2 5 nm,Light-emitting element 3 10 nm, Light-emitting element 4 0 nm **2mDBTBPDBq-II:PCBBiF:Ir(dmdppr-dmp)₂(acac) = 0.2:0.8:0.05 ***2mDBTBPDBq-II:PCBBiF:Ir(tBuppm)₂(acac) = 0.7:0.3:0.05

FIG. 13 shows current density-luminance characteristics ofLight-emitting elements 1 to 4. FIG. 14 shows luminance-currentefficiency characteristics Light-emitting elements 1 to 4. FIG. 15 showsvoltage-luminance characteristics of Light-emitting elements 1 to 4.FIG. 16 shows luminance-external quantum efficiency characteristics ofLight-emitting elements 1 to 4. FIG. 17 shows emission spectra ofLight-emitting elements 1 to 4.

As can be seen from the characteristics, although Light-emittingelements 1 to 4 did not include an intermediate layer, Light-emittingelements 1 to 4 had a current efficiency of 40 cd/A or higher at around1000 cd/m² and an external quantum efficiency of 18% or higher. Thisindicates that Light-emitting elements 1 to 4 had high emissionefficiency. The drive voltage at around 1000 cd/m² is as low as in the3-V range, which is much lower than the drive voltage of a tandemlight-emitting element. Moreover, Light-emitting elements 1 to 3 eachhave extremely favorable efficiency: external quantum efficiency of 19%or higher at around 1000 cd/m². Table 2 lists values of maincharacteristics of Light-emitting Elements 1 to 4 at around 1000 cd/m².

TABLE 2 Ex- ternal Cor- quan- related Current Current Power tum Energycolor Volt- density effi- effi- effi- effi- tem- age (mA/ ciency ciencyciency ciency perature (V) cm²) (cd/A) (lm/W) (%) (%) (K) duv Light- 3.11.8 47 48 20 14 2570 0.011 emitting element 1 Light- 3.2 2.0 43 42 19 132520 0.01 emitting element 2 Light- 3.3 2.8 40 38 19 12 2180 0.006emitting element 3 Light- 3.2 2.5 45 44 18 12 2930 0.016 emittingelement 4

Moreover, the emission spectrum in FIG. 17 shows that red light emissionoriginating from [Ir(dmdppr-dmp)₂(acac)], green light emissionoriginating from [Ir(tBuppm)₂(acac)], and blue light emissionoriginating from 1,6mMemFLPAPrn were observed. This indicates that lightemission was sufficiently obtained from both the first light-emittinglayer 113 a that was a fluorescent layer and the second light-emittinglayer 113 c that was a phosphorescent layer.

As described above, Light-emitting elements 1 to 4 had highlywell-balanced, favorable characteristics and was able to be fabricatedeasily and inexpensively. The above-described results were attributed tothe following: diffusion of excitons was suppressed and non-radiativedecay of the triplet excitation energy was reduced by using the exciplexas an energy donor of the phosphorescent layer, and the emissionefficiency was improved because of occurrence of delayed fluorescencedue to triplet-triplet annihilation in the host material in thefluorescent layer. The use of the separation layer 113 b suppressesenergy transfer from the phosphorescent layer (the second light-emittinglayer 113 c) to the fluorescent layer (the first light-emitting layer113 a) at their interface; therefore, Light-emitting elements 1 to 3 canhave more favorable characteristics.

Example 2

In this example, a method of fabricating Light-emitting element 5 whichis one embodiment of the present invention and characteristics thereofare described. Structural formulae of organic compounds used forLight-emitting element 5 are shown below.

(Method of Fabricating Light-Emitting Element 5)

A film of indium tin oxide containing silicon oxide (ITSO) was formedover a glass substrate by a sputtering method to form the firstelectrode 101. The thickness was 110 nm and the electrode area was 2mm×2 mm. Here, the first electrode 101 functions as an anode of thelight-emitting element.

Next, in pretreatment for forming the light-emitting element over thesubstrate, a surface of the substrate was washed with water and baked at200° C. for 1 hour, and then UV ozone treatment was performed for 370seconds.

After that, the substrate was transferred into a vacuum evaporationapparatus where the pressure had been reduced to approximately 10⁻⁴ Pa,and was subjected to vacuum baking at 170° C. for 30 minutes in aheating chamber of the vacuum evaporation apparatus, and then thesubstrate was cooled down for approximately 30 minutes.

Then, the substrate provided with the first electrode 101 was fixed to asubstrate holder provided in the vacuum evaporation apparatus so thatthe surface on which the first electrode 101 was formed faced downward.The pressure in the vacuum evaporation apparatus was reduced to about10⁻⁴ Pa. After that, on the first electrode 101,4,4′,4″-(benzene-1,3,5-triyl)tri(dibenzothiophene) (abbreviation:DBT3P-II) represented by Structural Formula (i) and molybdenum(VI) oxidewere deposited by co-evaporation by an evaporation method usingresistance heating to form the hole-injection layer 111. The thicknesswas set to 40 nm, and the weight ratio of DBT3P-II to molybdenum oxidewas adjusted to 4:2 (=DBT3P-II: molybdenum oxide). Note that theco-evaporation method refers to an evaporation method in whichevaporation is carried out from a plurality of evaporation sources atthe same time in one treatment chamber.

Next, on the hole-injection layer 111,3-[4-(9-phenanthryl)-phenyl]-9-phenyl-9H-carbazole (abbreviation: PCPPn)represented by Structural Formula (ii) was deposited to a thickness of10 nm; thus, the hole-transport layer 112 was formed.

On the hole-transport layer 112,7-[4-(10-phenyl-9-anthryl)phenyl]-7H-dibenzo[c,g]carbazole(abbreviation: cgDBCzPA) represented by Structural Formula (iii) andN,N′-bis(3-methylphenyl)-N,N′-bis[3-(9-phenyl-9H-fluoren-9-yl)phenyl]-pyrene-1,6-diamine(abbreviation: 1,6mMemFLPAPrn) represented by Structural Formula (iv)were deposited by co-evaporation to a thickness of 5 nm such that theweight ratio of cgDBCzPA to 1,6mMemFLPAPrn was 1:0.04 (=cgDBCzPA:1,6mMemFLPAPrn); thus, the first light-emitting layer 113 a that was afluorescent layer was formed. Next,2-[3′-(dibenzothiophen-4-yl)biphenyl-3-yl]dibenzo[a]quinoxaline(abbreviation: 2mDBTBPDBq-II) represented by Structural Formula (v) andN-(1,1′-biphenyl-4-yl)-N-[4-(9-phenyl-9H-carbazol-3-yl)phenyl]-9,9-dimethyl-9H-fluoren-2-amine(abbreviation: PCBBiF) represented by Structural Formula (vi) weredeposited by co-evaporation to a thickness of 2 nm such that the weightratio of 2mDBTBPDBq-II to PCBBiF was 0.6:0.4 (=2mDBTBPDBq-II:PCBBiF);thus, the separation layer 113 b was formed. Then, the firstphosphorescent layer 113 c-1 was formed in such a manner that2mDBTBPDBq-II, PCBBiF, andbis{4,6-dimethyl-2-[5-(2,6-dimethylphenyl)-3-(3,5-dimethylphenyl)-2-pyrazinyl-κN]phenyl-κC}(2,4-pentanedionato-κ²O,O′)iridium(III)(abbreviation: [Ir(dmdppr-dmp)₂(acac)]) represented by StructuralFormula (vii) were deposited by co-evaporation to a thickness of 5 nmsuch that the weight ratio of 2mDBTBPDBq-II to PCBBiF and[Ir(dmdppr-dmp)₂(acac)] was 0.5:0.5:0.05(=2mDBTBPDBq-II:PCBBiF:[Ir(dmdppr-dmp)₂(acac)]); the secondphosphorescent layer 113 c-2 was successively formed in such a mannerthat 2mDBTBPDBq-II, PCBBiF, andbis[2-(6-tert-butyl-4-pyrimidinyl-κN3)phenyl-κC](2,4-pentanedionato-κ²O,O′)iridium(III) (abbreviation: [Ir(tBuppm)₂(acac)]) represented by StructuralFormula (viii) were deposited by co-evaporation to a thickness of 20 nmsuch that the weight ratio of 2mDBTBPDBq-II to PCBBiF and[Ir(tBuppm)₂(acac)] was 0.7:0.3:0.05(=2mDBTBPDBq-II:PCBBiF:[Ir(tBuppm)₂(acac)]). Through the above steps,the second light-emitting layer 113 c that was a phosphorescent layerwas formed.

Note that 2mDBTBPDBq-II and PCBBiF form an exciplex in thephosphorescent layer (the second light-emitting layer 113 c). Thisemission wavelength overlaps absorption bands on the longest wavelengthsides of [Ir(dmdppr-dmp)₂(acac)] and [Ir(tBuppm)₂(acac)], so that energytransfer efficiency is high.

The singlet excitation energy of cgDBCzPA that is a host material in thefluorescent layer (the first light-emitting layer 113 a) is higher thanthe singlet excitation energy of 1,6mMemFLPAPrn that is a fluorescentsubstance. In addition, the triplet excitation energy of cgDBCzPA islower than the triplet excitation energy of 1,6mMemFLPAPrn. Thus, in thefluorescent layer (the first light-emitting layer 113 a), regenerationof a singlet exciton associated with triplet-triplet annihilation andlight emission are obtained easily.

After that, on the phosphorescent layer (the second light-emitting layer113 c), 2mDBTBPDBq-II was deposited to a thickness of 10 nm, andbathophenanthroline (abbreviation: BPhen) represented by StructuralFormula (ix) was deposited to a thickness of 15 nm to form theelectron-transport layer 114.

After the formation of the electron-transport layer 114, lithiumfluoride (LiF) was deposited by evaporation to a thickness of 1 nm toform the electron-injection layer 115. Finally, aluminum was depositedby evaporation to a thickness of 200 nm to form the second electrode 102functioning as a cathode. Through the above-described steps,Light-emitting element 5 of this example was fabricated.

Note that in all the above evaporation steps, evaporation was performedby a resistance-heating method.

In a glove box under a nitrogen atmosphere, Light-emitting element 5 wassealed with a glass substrate so as not to be exposed to the air(specifically, a sealant was applied onto an outer edge of the element,and at the time of sealing, UV treatment was performed and then heattreatment was performed at 80° C. for 1 hour). Then, the reliability ofthe light-emitting element was measured. Note that the measurement wasperformed at room temperature (in the atmosphere kept at 25° C.).

Table 3 shows the element structures of Light-emitting element 5.

TABLE 3 Hole- Hole- First light- Electron- Electron- injection transportemitting Separation Second light- transport injection layer layer layerlayer emitting layer layer layer DBT3P- PCPPn cgDBCzPA: * ** *** 2mDBTBPhen LiF II:MoOx 10 nm 1,6mMem 2 nm 5 nm 20 nm BPDBq-II 15 nm 1 nm 2:1FLPAPm 10 nm 40 nm 1:0.04 5 nm * 2mDBTBPDBq-II:PCBBiF = 0.6:0.4 **2mDBTBPDBq-II:PCBBiF:Ir(dmdppr-dmp)₂(acac) = 0.5:0.5:0.05 ***2mDBTBPDBq-II:PCBBiF:Ir(tBuppm)₂(acac) = 0.7:0.3:0.05

As for Light-emitting element 5, FIG. 18 shows the currentdensity-luminance characteristics, FIG. 19 shows the luminance-currentefficiency characteristics, FIG. 20 shows the voltage-luminancecharacteristics, FIG. 21 shows the luminance-external quantum efficiencycharacteristics, FIG. 22 shows the emission spectrum, and FIG. 23 showsthe luminance-CIE chromaticity characteristics.

As can be seen from the characteristics, although Light-emitting element5 did not include an intermediate layer, Light-emitting element 5 had acurrent efficiency of 40 cd/A or higher at around 1000 cd/m² and anexternal quantum efficiency of 18% or higher. This indicates thatLight-emitting element 5 had high emission efficiency. The drive voltageis as low as in the 3-V range, which is much lower than the drivevoltage of a tandem light-emitting element. Table 4 lists values of maincharacteristics of Light-emitting Element 5 at around 1000 cd/m².

TABLE 4 Ex- Cor- ternal related Current Current Power quantum Energycolor Volt- density effi- effi- effi- effi- tem- age (mA/ ciency ciencyciency ciency perature (V) cm²) (cd/A) (lm/W) (%) (%) (K) duv Light- 3.22.5 46 46 18 12 3010 0.017 emitting element 5

Moreover, the emission spectrum in FIG. 22 shows that red light emissionoriginating from [Ir(dmdppr-dmp)₂(acac)], green light emissionoriginating from [Ir(tBuppm)₂(acac)], and blue light emissionoriginating from 1,6mMemFLPAPrn were observed. This indicates that lightemission was sufficiently obtained from both the first light-emittinglayer 113 a that was a fluorescent layer and the second light-emittinglayer 113 c that was a phosphorescent layer.

The luminance-CIE chromaticity characteristics shown in FIG. 23 indicatethat Light-emitting element 5 has an extremely small color change in thepractical luminance region. Note that the color change at around 100cd/m² is due to a difference between light emission start voltages ofphosphorescence and fluorescence. Since the phosphorescent layer has alower light emission start voltage than the fluorescent layer, onlyphosphorescence is observed in a low luminance region, and fluorescenceis also observed at around 100 cd/m². For this reason, the color changeoccurs at around 100 cd/m². In the practical luminance region wherefluorescence and phosphorescence are both stable, color change isextremely small.

As described above, Light-emitting element 5 had highly well-balanced,favorable characteristics and was able to be fabricated easily andinexpensively. The above-described results were attributed to thefollowing: diffusion of excitons was suppressed and non-radiative decayof the triplet excitation energy was reduced by using the exciplex as anenergy donor of the phosphorescent layer, and the emission efficiencywas improved because of occurrence of delayed fluorescence due totriplet-triplet annihilation in the host material in the fluorescentlayer. The use of the separation layer 113 b suppresses energy transferfrom the phosphorescent layer (the second light-emitting layer 113 c) tothe fluorescent layer (the first light-emitting layer 113 a) at theirinterface; therefore, Light-emitting element 5 can have more favorablecharacteristics.

Example 3

In this example, methods of fabricating Light-emitting elements 6 and 7each of which is one embodiment of the present invention andcharacteristics thereof are described. Structural formulae of organiccompounds used for Light-emitting elements 6 and 7 are shown below.

(Method of Fabricating Light-Emitting Element 6)

A film of indium tin oxide containing silicon oxide (ITSO) was formedover a glass substrate by a sputtering method to form the firstelectrode 101. The thickness was 110 nm and the electrode area was 2mm×2 mm. Here, the first electrode 101 functions as an anode of thelight-emitting element.

Next, in pretreatment for forming the light-emitting element over thesubstrate, a surface of the substrate was washed with water and baked at200° C. for 1 hour, and then UV ozone treatment was performed for 370seconds.

After that, the substrate was transferred into a vacuum evaporationapparatus where the pressure had been reduced to approximately 10⁻⁴ Pa,and was subjected to vacuum baking at 170° C. for 30 minutes in aheating chamber of the vacuum evaporation apparatus, and then thesubstrate was cooled down for approximately 30 minutes.

Then, the substrate provided with the first electrode 101 was fixed to asubstrate holder provided in the vacuum evaporation apparatus so thatthe surface on which the first electrode 101 was formed faced downward.The pressure in the vacuum evaporation apparatus was reduced to about10⁻⁴ Pa. After that, on the first electrode 101,4,4′,4″-(benzene-1,3,5-triyl)tri(dibenzothiophene) (abbreviation:DBT3P-II) represented by Structural Formula (i) and molybdenum(VI) oxidewere deposited by co-evaporation by an evaporation method usingresistance heating to form the hole-injection layer 111. The thicknesswas set to 30 nm, and the weight ratio of DBT3P-II to molybdenum oxidewas adjusted to 2:1 (=DBT3P-II:molybdenum oxide).

Next, on the hole-injection layer 111,9-phenyl-3-[4-(10-phenyl-9-anthryl)phenyl]-9H-carbazole (abbreviation:PCzPA) represented by Structural Formula (x) was deposited to athickness of 20 nm; thus, the hole-transport layer 112 was formed.

On the hole-transport layer 112, PCzPA and 1,6mMemFLPAPrn were depositedby co-evaporation to a thickness of 5 nm such that the weight ratio ofPCzPA to 1,6mMemFLPAPrn was 1:0.05 (=PCzPA:1,6mMemFLPAPrn); thus, thefirst light-emitting layer 113 a that was a fluorescent layer wasformed. Next, 4,6-bis[3-(9H-carbazol-9-yl)phenyl]pyrimidine(abbreviation: 4,6mCzP2Pm) represented by Structural Formula (xi) and4,4′-di(N-carbazolyl)biphenyl (abbreviation: CBP) represented byStructural Formula (xii) were deposited by co-evaporation to a thicknessof 2 nm such that the weight ratio of 4,6mCzP2Pm to CBP was 0.4:0.6(=4,6mCzP2Pm:CBP); thus, the separation layer 113 b was formed. Then,2mDBTBPDBq-II, PCBBiF, and(acetylacetonato)bis(4,6-diphenylpyrimidinato)iridium(III)(abbreviation: [Ir(dppm)₂(acac)]) represented by Structural Formula(xiii) were deposited by co-evaporation to a thickness of 20 nm suchthat the weight ratio of 2mDBTBPDBq-II to PCBBiF and [Ir(dppm)₂(acac)]was 0.8:0.2:0.05 (=2mDBTBPDBq-II:PCBBiF:[Ir(dppm)₂(acac)]); thus, thesecond light-emitting layer 113 c that was a phosphorescent layer wasformed.

Note that 2mDBTBPDBq-II and PCBBiF form an exciplex in thephosphorescent layer (the second light-emitting layer 113 c). Thisemission wavelength overlaps absorption bands on the longest wavelengthside of [Ir(dppm)₂(acac)], so that energy transfer efficiency is high.

The singlet excitation energy of PCzPA that is a host material in thefluorescent layer (the first light-emitting layer 113 a) is higher thanthe singlet excitation energy of 1,6mMemFLPAPrn that is a fluorescentsubstance. In addition, the triplet excitation energy of PCzPA is lowerthan the triplet excitation energy of 1,6mMemFLPAPrn. Thus, in thefluorescent layer (the first light-emitting layer 113 a), regenerationof a singlet exciton associated with triplet-triplet annihilation andlight emission are obtained easily.

After that, on the phosphorescent layer (the second light-emitting layer113 c), 2mDBTBPDBq-II was deposited to a thickness of 10 nm, and BPhenwas deposited to a thickness of 15 nm to form the electron-transportlayer 114.

After the formation of the electron-transport layer 114, lithiumfluoride (LiF) was deposited by evaporation to a thickness of 1 nm toform the electron-injection layer 115. Finally, aluminum was depositedby evaporation to a thickness of 200 nm to form the second electrode 102functioning as a cathode. Through the above-described steps,Light-emitting element 6 of this example was fabricated.

Note that in all the above evaporation steps, evaporation was performedby a resistance-heating method.

(Method of Fabricating Light-Emitting Element 7)

Light-emitting element 7 was fabricated in a manner similar to that ofLight-emitting element 6 except that the separation layer 113 b wasformed with 4,6mCzP2Pm alone.

In a glove box under a nitrogen atmosphere, each of Light-emittingelements 6 and 7 was sealed with a glass substrate so as not to beexposed to the air (specifically, a sealant was applied onto an outeredge of the element, and at the time of sealing, IN treatment wasperformed and then heat treatment was performed at 80° C. for 1 hour).Then, the reliability of these light-emitting elements was measured.Note that the measurement was performed at room temperature (in theatmosphere kept at 25° C.).

Table 5 shows the element structures of Light-emitting elements 6 and 7.

TABLE 5 Hole- Hole- First light- Electron- Electron- injection transportemitting Separation Second light- transport injection layer layer layerlayer emitting layer layer layer DBT3P- PCzPA PCzPA: * 2mDBT 2mDBT BPhenLiF II:MoOx 20 nm 1,6mMem 2 nm BPDBq-II: BPDBq-II 15 nm 1 nm 2:1 FLPAPmPCBBiF: 10 nm 30 nm 1:0.05 Ir(dppm)₂(acac) 10 nm 0.8:0.2:0.05 20 nm *Light-emitting element 6 4,6mCzP2Pm:CBP = 0.4:0.6 Light-emitting element7 4,6mCzP2Pm

FIG. 24 shows current density-luminance characteristics ofLight-emitting elements 6 and 7. FIG. 25 shows luminance-currentefficiency characteristics Light-emitting elements 6 and 7. FIG. 26shows voltage-luminance characteristics of Light-emitting elements 6 and7. FIG. 27 shows luminance-external quantum efficiency characteristicsof Light-emitting elements 6 and 7. FIG. 28 shows emission spectra ofLight-emitting elements 6 and 7.

The above results show that Light-emitting element 6 and Light-emittingelement 7 each have favorable emission efficiency of a currentefficiency of 20 cd/A or higher at around 1000 cd/m². In addition, thedrive voltage is in the 3-V range, which is much lower than that of atandem light-emitting element. Table 6 lists values of maincharacteristics of Light-emitting Elements 6 and 7 at around 1000 cd/m².

TABLE 6 Cor- External related Current Current Power quantum Energy colorVolt- density effi- effi- effi- effi- tem- age (mA/ ciency ciency ciencyciency perature (V) cm²) (cd/A) (lm/W) (%) (%) (K) duv Light- 3.1 1.7 3132 12 9 2710 0.003 emitting element 6 Light- 3.3 4.8 23 22 10 6 35400.009 emitting element 7

Moreover, the emission spectrum shows that orange light emissionoriginating from [Ir(dppm)₂(acac)] and blue light emission originatingfrom 1,6mMemFLPAPrn were observed. This indicates that in Light-emittingelements 6 and 7, light emission was sufficiently obtained from both thefirst light-emitting layer 113 a that was a fluorescent layer and thesecond light-emitting layer 113 c that was a phosphorescent layer.

As described above, Light-emitting elements 6 and 7 have highlywell-balanced favorable characteristics and can be fabricated easily andinexpensively. The above-described results were attributed to thefollowing: diffusion of excitons was suppressed and non-radiative decayof the triplet excitation energy was reduced by using the exciplex as anenergy donor of the phosphorescent layer, and the emission efficiencywas improved because of occurrence of delayed fluorescence due totriplet-triplet annihilation in the host material of the fluorescentlayer. In addition, the use of the separation layer 113 b suppressesenergy transfer from the phosphorescent layer (the second light-emittinglayer 113 c) to the fluorescent layer (the first light-emitting layer113 a) at their interface, which is also a reason of the favorablecharacteristics. Light-emitting element 6 has better characteristicsthan Light-emitting element 7; therefore, the separation layer 113 b ispreferably formed with a substance having a hole-transport property anda substance having an electron-transport property. Moreover, it is morepreferable that these substances form an exciplex.

Example 4

In this example, a method of fabricating Light-emitting element 8 whichis one embodiment of the present invention and characteristics thereofare described. In Light-emitting element 8, the first light-emittinglayer 113 a was formed on the cathode side and the second light-emittinglayer 113 c was formed on the anode side. Structural formulae of organiccompounds used for Light-emitting element 8 are shown below.

(Method of Fabricating Light-Emitting Element 8)

A film of indium tin oxide (ITO) was deposited over a glass substratewith a high refractive index (n=1.84) to a thickness of 110 nm by asputtering method to form the first electrode 101. The electrode areawas 2 mm×2 mm.

Next, in pretreatment for forming the light-emitting element over thesubstrate, a surface of the substrate was washed with water and then UVozone treatment was performed for 370 seconds.

After that, the substrate was transferred into a vacuum evaporationapparatus where the pressure had been reduced to approximately 10⁻⁴ Pa,and was subjected to vacuum baking at 190° C. for 60 minutes in aheating chamber of the vacuum evaporation apparatus, and then thesubstrate was cooled down for approximately 30 minutes.

Then, the substrate provided with the first electrode 101 was fixed to asubstrate holder provided in the vacuum evaporation apparatus so thatthe surface on which the first electrode 101 was formed faced downward.The pressure in the vacuum evaporation apparatus was reduced to about10⁻⁴ Pa. After that, on the first electrode 101,4,4′,4″-(benzene-1,3,5-triyl)tri(dibenzothiophene) (abbreviation:DBT3P-II) represented by Structural Formula (i) and molybdenum(VI) oxidewere deposited by co-evaporation by an evaporation method usingresistance heating to form the hole-injection layer 111. The thicknesswas set to 30 nm, and the weight ratio of DBT3P-II to molybdenum oxidewas adjusted to 1:0.5 (=DBT3P-II: molybdenum oxide).

Next, on the hole-injection layer 111,N-(1,1′-biphenyl-4-yl)-N-[4-(9-phenyl-9H-carbazol-3-yl)phenyl]-9,9-dimethyl-9H-fluoren-2-amine(abbreviation: PCBBiF) represented by Structural Formula (vi) wasdeposited to a thickness of 20 nm; thus, the hole-transport layer 112was formed.

Then, on the hole-transport layer 112, the first phosphorescent layer113 c-1 was formed in such a manner that2-[3′-(dibenzothiophen-4-yl)biphenyl-3-yl]dibenzo[f,h]quinoxaline(abbreviation: 2mDBTBPDBq-II) represented by Structural Formula (v),N-(1,1′-biphenyl-4-yl)-N-[4-(9-phenyl-9H-carbazol-3-yl)phenyl]-9,9-dimethyl-9H-fluoren-2-amine(abbreviation: PCBBiF) represented by Structural Formula (vi), andbis{4,6-dimethyl-2-[5-(2,6-dimethylphenyl)-3-(3,5-dimethylphenyl)-2-pyrazinyl-κN]phenyl-κC}(2,4-pentanedionato-κ²O,O′)iridium(III)(abbreviation: [Ir(dmdppr-dmp)₂(acac)]) represented by StructuralFormula (vii) were deposited by co-evaporation to a thickness of 15 nmsuch that the weight ratio of 2mDBTBPDBq-II to PCBBiF and[Ir(dmdppr-dmp)₂(acac)] was 0.1:0.9:0.06(=2mDBTBPDBq-II:PCBBiF:[Ir(dmdppr-dmp)₂(acac)]); the secondphosphorescent layer 113 c-2 was successively formed in such a mannerthat 2mDBTBPDBq-II, PCBBiF, andbis[2-(6-tert-butyl-4-pyrimidinyl-κN3)phenyl-κC](2,4-pentanedionato-κ²O,O′)iridium(III)(abbreviation: [Ir(tBuppm)₂(acac)]) represented by Structural Formula(viii) were deposited by co-evaporation to a thickness of 5 nm such thatthe weight ratio of 2mDBTBPDBq-II to PCBBiF and [Ir(tBuppm)₂(acac)] was0.5:0.5:0.06 (=2mDBTBPDBq-II:PCBBiF:[Ir(tBuppm)₂(acac)]). Through theabove steps, the second light-emitting layer 113 c that was aphosphorescent layer was formed. After that, 2mDBTBPDBq-II and PCBBiFwere deposited by co-evaporation to a thickness of 2 nm such that theweight ratio of 2mDBTBPDBq-II to PCBBiF was 0.5:0.5(=2mDBTBPDBq-II:PCBBiF); thus, the separation layer 113 b was formed.Then, 7-[4-(10-phenyl-9-anthryl)phenyl]-7H-dibenzo[c,g]carbazole(abbreviation: cgDBCzPA) represented by Structural Formula (iii) andN,N′-bis(3-methylphenyl)-N,N′-bis[3-(9-phenyl-9H-fluoren-9-yl)phenyl]pyrene-1,6-diamine(abbreviation: 1,6mMemFLPAPrn) represented by Structural Formula (iv)were deposited by co-evaporation to a thickness of 20 nm such that theweight ratio of cgDBCzPA to 1,6mMemFLPAPrn was 1:0.025(=cgDBCzPA:1,6mMemFLPAPrn); thus, the first light-emitting layer 113 athat was a fluorescent layer was formed. Through the above-describedsteps, the light-emitting layer 113 was formed.

Note that 2mDBTBPDBq-II and PCBBiF form an exciplex in thephosphorescent layer (the second light-emitting layer 113 c) and theseparation layer 113 b. This emission wavelength overlaps absorptionbands on the longest wavelength sides of [Ir(dmdppr-dmp)₂(acac)] and[Ir(tBuppm)₂(acac)], so that energy transfer efficiency is high.

The singlet excitation energy of cgDBCzPA that is a host material in thefluorescent layer (the first light-emitting layer 113 a) is higher thanthe singlet excitation energy of 1,6mMemFLPAPrn that is a fluorescentsubstance. In addition, the triplet excitation energy of cgDBCzPA islower than the triplet excitation energy of 1,6mMemFLPAPrn. Thus, in thefluorescent layer (the first light-emitting layer 113 a), regenerationof a singlet exciton associated with triplet-triplet annihilation andlight emission are obtained easily.

After that, on the phosphorescent layer (the first light-emitting layer113 a), cgDBCzPA was deposited to a thickness of 10 nm, andbathophenanthroline (abbreviation: BPhen) represented by StructuralFormula (ix) was deposited to a thickness of 15 nm to form theelectron-transport layer 114.

After the formation of the electron-transport layer 114, lithiumfluoride (LiF) was deposited by evaporation to a thickness of 1 nm toform the electron-injection layer 115. Finally, an alloy of silver andmagnesium (1:0.5) was deposited to a thickness of 1 nm and silver wasdeposited to a thickness of 150 nm by evaporation to form the secondelectrode 102 functioning as a cathode. Through the above-describedsteps, Light-emitting element 8 of this example was fabricated. Notethat in all the above evaporation steps, evaporation was performed by aresistance-heating method.

Table 7 shows the element structure of Light-emitting element 8.

TABLE 7 Hole- Hole- Electron- Electron- injection transport Secondlight- Separation First light- transport injection layer layer emittinglayer layer emitting layer layer layer DBT3P- PCBBiF * ** *** cgDBCzPA:cgDBCzPA BPhen LiF II:MoOx 20 nm 15 nm 5 nm 2 nm 1,6mMem 15 nm 1 nm1:0.5 FLPAPm 30 nm 1:0.03 5 nm *2mDBTBPDBq-II:PCBBiF:Ir(dmdppr-dmp)₂(acac) = 0.1:0.9:0.06 **2mDBTBPDBq-II:PCBBiF:Ir(tBuppm)₂(acac) = 0.5:0.5:0.06 ***2mDBTBPDBq-II:PCBBiF = 0.5:0.5

In a glove box under a nitrogen atmosphere, Light-emitting element 8 wassealed with a glass substrate so as not to be exposed to the air(specifically, a sealant was applied onto an outer edge of the element,and at the time of sealing, UV treatment was performed and then heattreatment was performed at 80° C. for 1 hour). Then, the characteristicsof the light-emitting element were measured. Note that the measurementwas performed with an integrating sphere at room temperature (in theatmosphere kept at 25° C.). Table 8 shows values of the characteristicsat a current density of 3.75 mA/cm².

TABLE 8 Cor- General Ex- related color ternal color ren- Power quantumVolt- tem- dering effi- effi- age perature index ciency ciency (V) (K)duv Ra (lm/W) (%) Light- 2.8 2840 0.0149 87 57 21 emitting element 8

Light-emitting element 8 exhibited favorable external quantum efficiencyand power efficiency. Light-emitting element 8 had a very low drivevoltage of 2.8 V as compared with a tandem light-emitting element.

FIG. 29 shows the emission spectrum of Light-emitting element 8. As canbe seen from the emission spectrum, red light emission originating from[Ir(dmdppr-dmp)₂(acac)], green light emission originating from[Ir(tBuppm)₂(acac)], and blue light emission originating from1,6mMemFLPAPrn were observed. This indicates that light emission wassufficiently obtained from both the first light-emitting layer 113 athat was a fluorescent layer and the second light-emitting layer 113 cthat was a phosphorescent layer.

Light-emitting element 8 had a general color rendering index (Ra) of 87,which means that Light-emitting element 8 had a favorable colorrendering property, and had small duv (a deviation from blackbodyradiation locus); thus, Light-emitting element 8 is suitably used forlighting. Furthermore, Light-emitting element 8 had a color temperatureof 2840 K that is incandescent color.

As described above, Light-emitting element 8 had well-balanced,favorable characteristics and was able to be fabricated easily andinexpensively. The above-described results were attributed to thefollowing: diffusion of excitons was suppressed and non-radiative decayof the triplet excitation energy was reduced by using the exciplex as anenergy donor of the phosphorescent layer, and the emission efficiencywas improved because of occurrence of delayed fluorescence due totriplet-triplet annihilation in the host material in the fluorescentlayer.

Example 5

In this example, a method of fabricating Light-emitting element 9 whichis one embodiment of the present invention and characteristics thereofare described. Structural formulae of organic compounds used forLight-emitting element 9 are shown below.

(Method of Fabricating Light-Emitting Element 9)

A film of indium tin oxide containing silicon oxide (ITSO) was formedover a glass substrate by a sputtering method to form the firstelectrode 101. The thickness was 110 nm and the electrode area was 2mm×2 mm. Here, the first electrode 101 functions as an anode of thelight-emitting element.

Next, in pretreatment for forming the light-emitting element over thesubstrate, a surface of the substrate was washed with water and baked at200° C. for 1 hour, and then UV ozone treatment was performed for 370seconds.

After that, the substrate was transferred into a vacuum evaporationapparatus where the pressure had been reduced to approximately 10⁻⁴ Pa,and was subjected to vacuum baking at 170° C. for 30 minutes in aheating chamber of the vacuum evaporation apparatus, and then thesubstrate was cooled down for approximately 30 minutes.

Then, the substrate provided with the first electrode 101 was fixed to asubstrate holder provided in the vacuum evaporation apparatus so thatthe surface on which the first electrode 101 was formed faced downward.The pressure in the vacuum evaporation apparatus was reduced to about10⁻⁴ Pa. After that, on the first electrode 101,4,4′,4″-(benzene-1,3,5-triyl)tri(dibenzothiophene) (abbreviation:DBT3P-II) represented by Structural Formula (i) and molybdenum(VI) oxidewere deposited by co-evaporation by an evaporation method usingresistance heating to form the hole-injection layer 111. The thicknesswas set to 15 nm, and the weight ratio of DBT3P-II to molybdenum oxidewas adjusted to 2:1 (=DBT3P-II: molybdenum oxide).

Next, on the hole-injection layer 111,3-[4-(9-phenanthryl)-phenyl]-9-phenyl-9H-carbazole (abbreviation: PCPPn)represented by Structural Formula (ii) was deposited to a thickness of20 nm; thus, the hole-transport layer 112 was formed.

Then, on the hole-transport layer 112,7-[4-(10-phenyl-9-anthryl)phenyl]-7H-dibenzo[c,g]carbazole(abbreviation: cgDBCzPA) represented by Structural Formula (iii) andN,N′-bis(3-methylphenyl)-N,N′-bis[3-(9-phenyl-9H-fluoren-9-yl)phenyl]-pyrene-1,6-diamine(abbreviation: 1,6mMemFLPAPrn) represented by Structural Formula (iv)were deposited by co-evaporation to a thickness of 5 nm such that theweight ratio of cgDBCzPA to 1,6mMemFLPAPrn was 2:0.1(=cgDBCzPA:1,6mMemFLPAPrn); thus, the first light-emitting layer 113 athat was a fluorescent layer was formed. Next,2-[3′-(dibenzothiophen-4-yl)biphenyl-3-yl]dibenzo[f,h]quinoxaline(abbreviation: 2mDBTBPDBq-II) represented by Structural Formula (v) andN-(1,1′-biphenyl-4-yl)-N-[4-(9-phenyl-9H-carbazol-3-yl)phenyl]-9,9-dimethyl-9H-fluoren-2-amine(abbreviation: PCBBiF) represented by Structural Formula (vi) weredeposited by co-evaporation to a thickness of 2 nm such that the weightratio of 2mDBTBPDBq-II to PCBBiF was 0.4:1.6 (=2mDBTBPDBq-II:PCBBiF);thus, the separation layer 113 b was formed. After that, the firstphosphorescent layer 113 c-1 was formed in such a manner that2mDBTBPDBq-II, PCBBiF, and(acetylacetonato)bis(4,6-diphenylpyrimidinato)iridium(III)(abbreviation: [Ir(dppm)₂(acac)]) represented by Structural Formula(xiv) were deposited by co-evaporation to a thickness of 5 nm such thatthe weight ratio of 2mDBTBPDBq-II to PCBBiF and [Ir(dppm)₂(acac)] was0.8:1.2:0.12 (=2mDBTBPDBq-II:PCBBiF:[Ir(dppm)₂(acac)]); the secondphosphorescent layer 113 c-2 was successively formed in such a mannerthat 2mDBTBPDBq-II, PCBBiF, andbis{2-[6-(2,6-dimethylphenyl)-4-pyrimidinyl-κN3]phenyl-κC}(2,4-pentanedionato-κO,O′)iridium(III) (abbreviation: [Ir(ppm-dmp)₂(acac)])represented by Structural Formula (xv) were deposited by co-evaporationto a thickness of 15 nm such that the weight ratio of 2mDBTBPDBq-II toPCBBiF and [Ir(ppm-dmp)₂(acac)] was 1.6:0.4:0.12(=2mDBTBPDBq-II:PCBBiF:[ft(ppm-dmp)₂(acac)]). Through the above steps,the second light-emitting layer 113 c that was a phosphorescent layerwas formed. As described above, the light-emitting layer 113 inLight-emitting element 9 was formed of the first light-emitting layer113 a and the second light-emitting layer 113 c.

Note that 2mDBTBPDBq-II and PCBBiF form an exciplex in thephosphorescent layer (the second light-emitting layer 113 c). Thisemission wavelength overlaps absorption bands on the longest wavelengthsides of [Ir(dppm)₂(acac)] and [Ir(ppm-dmp)₂(acac)], so that energytransfer efficiency is high.

The singlet excitation energy of cgDBCzPA that is a host material in thefluorescent layer (the first light-emitting layer 113 a) is higher thanthe singlet excitation energy of 1,6mMemFLPAPrn that is a fluorescentsubstance. In addition, the triplet excitation energy of cgDBCzPA islower than the triplet excitation energy of 1,6mMemFLPAPrn. Thus, in thefluorescent layer (the first light-emitting layer 113 a), regenerationof a singlet exciton associated with triplet-triplet annihilation andlight emission are obtained easily.

After that, on the phosphorescent layer (the second light-emitting layer113 c), 2mDBTBPDBq-II was deposited to a thickness of 10 nm, andbathophenanthroline (abbreviation: BPhen) represented by StructuralFormula (ix) was deposited to a thickness of 15 nm to form theelectron-transport layer 114.

After the formation of the electron-transport layer 114, lithiumfluoride (LiF) was deposited by evaporation to a thickness of 1 nm;thus, the electron-injection layer 115 was formed. Finally, silver (Ag)and magnesium (Mg) were deposited by co-evaporation to a thickness of 1nm such that the ratio of silver to magnesium was 1:0.5; thus, thesecond electrode 102 functioning as a cathode was formed. Then, silverwas deposited to a thickness of 150 nm by a sputtering method. Throughthe above steps, Light-emitting element 9 of this example wasfabricated.

Note that in all the above evaporation steps, evaporation was performedby a resistance-heating method.

In a glove box under a nitrogen atmosphere, Light-emitting element 9 wassealed with a glass substrate so as not to be exposed to the air(specifically, a sealant was applied onto an outer edge of the element,and at the time of sealing, UV treatment was performed and then heattreatment was performed at 80° C. for 1 hour). Then, the reliability ofthe light-emitting element was measured. Note that the measurement wasperformed at room temperature (in the atmosphere kept at 25° C.).

Table 9 shows the element structure of Light-emitting element 9.

TABLE 9 Hole- Hole- First Electron- Electron- injection transportlight-emitting Separation Second light- transport injection layer layerlayer layer emitting layer layer layer DBT3P- PCPPn cgDBCzPA: * ** ***2mDBT BPhen LiF II:MoOx 20 nm 1,6mMem 2 nm 5 nm 15 nm BPDBq-II 15 nm 1nm 2:1 FLPAPm 10 nm 15 nm 2:0.1 5 nm * 2mDBTBPDBq-II:PCBBiF = 0.4:1.6 **2mDBTBPDBq-II:PCBBiF:Ir(dppm)₂(acac) = 0.8: 1.2:0.12 ***2mDBTBPDBq-II:PCBBiF:Ir(ppm-dmp)₂(acac) = 1.6:0.4:0.12

FIG. 30 shows current density-luminance characteristics ofLight-emitting element 9. FIG. 31 shows luminance-current efficiencycharacteristics Light-emitting element 9. FIG. 32 showsvoltage-luminance characteristics of Light-emitting element 9. FIG. 33shows luminance-external quantum efficiency characteristics ofLight-emitting element 9. FIG. 34 shows emission spectra ofLight-emitting element 9.

As can be seen from the characteristics, although Light-emitting element9 did not include an intermediate layer, Light-emitting element 9 had acurrent efficiency of approximately 70 cd/A at around 1000 cd/m² and anexternal quantum efficiency of 22% or higher. This indicates thatLight-emitting element 9 had very high emission efficiency. The drivevoltage is 2.9 V, which is much lower than the drive voltage of a tandemlight-emitting element.

TABLE 10 External Correlated Current Current Power quantum color Volt-density effi- effi- effi- tem- age (mA/ ciency ciency ciency perature(V) cm²) (cd/A) (lm/W) (%) (K) Light- 2.9 1.3 69 75 22 2730 emittingelement 9

As can be seen from the emission spectrum, phosphorescence originatingfrom [Ir(dppm)₂(acac)] and [Ir(ppm-dmp)₂(acac)] and fluorescenceoriginating from 1,6mMemFLPAPrn were both observed. This indicates thatlight emission was sufficiently obtained from both the firstlight-emitting layer 113 a that was the fluorescent layer and the secondlight-emitting layer 113 c that was the phosphorescent layer.

As described above, Light-emitting element 9 exhibits white lightemission with extremely high emission efficiency and was able to befabricated easily and inexpensively. The results were attributed to thefollowing: diffusion of excitons was suppressed and non-radiative decayof the triplet excitation energy was reduced by using the exciplex as anenergy donor of the phosphorescent layer, and the emission efficiencywas improved because of occurrence of delayed fluorescence due totriplet-triplet annihilation in the host material of the fluorescentlayer. The use of the separation layer 113 b suppresses energy transferfrom the phosphorescent layer (the second light-emitting layer 113 c) tothe fluorescent layer (the first light-emitting layer 113 a) at theirinterface; therefore, Light-emitting element 9 can have more favorablecharacteristics.

An organic EL lighting device was manufactured in the following manner:an element with a structure which is similar to the structure ofLight-emitting element 9 and which includes a 70-nm-thick firstelectrode was formed over a glass substrate with a refractive index of1.84 such that an emission area became 90 mm×90 mm; and a surface of thesubstrate from which light is emitted was frosted.

FIG. 35 shows the luminance-power efficiency characteristics of theorganic EL lighting device. The organic EL lighting device had a colortemperature of 2700K and duv=0.019, which confirm to the standards ofincandescent color, and had an extremely high power efficiency of 140lm/W at a luminance of around 1500 cd/m².

EXPLANATION OF REFERENCE

101: first electrode, 102: second electrode, 103: EL layer, 111:hole-injection layer, 112: hole-transport layer, 113: light-emittinglayer, 113 a: first light-emitting layer, 113 b: separation layer, 113c: second light-emitting layer, 113 c-1: first phosphorescent layer, 113c-2: second phosphorescent layer, 114: electron-transport layer, 115:electron-injection layer, 400: substrate, 401: first electrode, 403: ELlayer, 404: second electrode, 405: sealant, 406: sealant, 407: sealingsubstrate, 412: pad, 420: IC chip, 501: first electrode, 502: secondelectrode, 511: first light-emitting unit, 512: second light-emittingunit, 513: charge-generation layer, 601: driver circuit portion (sourceline driver circuit), 602: pixel portion, 603: driver circuit portion(gate line driver circuit), 604: sealing substrate, 605: sealant, 607:space, 608: wiring, 609: FPC (flexible printed circuit), 610: elementsubstrate, 611: switching FET, 612: current control FET, 613: firstelectrode, 614: insulator, 616: EL layer, 617: second electrode, 618:light-emitting element, 623: n-channel FET, 624: p-channel FET, 901:housing, 902: liquid crystal layer, 903: backlight unit, 904: housing,905: driver IC, 906: terminal, 951: substrate, 952: electrode, 953:insulating layer, 954: partition wall layer, 955: EL layer, 956:electrode, 1001: substrate, 1002: base insulating film, 1003: gateinsulating film, 1006: gate electrode, 1007: gate electrode, 1008: gateelectrode, 1020: first interlayer insulating film, 1021: secondinterlayer insulating film, 1022: electrode, 1024W: first electrode oflight-emitting element, 1024R: first electrode of light-emittingelement, 1024G: first electrode of light-emitting element, 1024B: firstelectrode of light-emitting element, 1025: partition wall, 1028: ELlayer, 1029: second electrode of light-emitting element, 1031: sealingsubstrate, 1032: sealant, 1033: transparent base material, 1034R: redcoloring layer, 1034G: green coloring layer, 1034B: blue coloring layer,1035: black layer (black matrix), 1036: overcoat layer, 1037: thirdinterlayer insulating film, 1040: pixel portion, 1041: driver circuitportion, 1042: peripheral portion, 2001: housing, 2002: light source,3001: lighting device, 5000: display region, 5001: display region, 5002:display region, 5003: display region, 5004: display region, 5005:display region, 7101: housing, 7103: display portion, 7105: stand, 7107:display portion, 7109: operation key, 7110: remote controller, 7201:main body, 7202: housing, 7203: display portion, 7204: keyboard, 7205:external connection port, 7206: pointing device, 7210: second displayportion, 7301: housing, 7302: housing, 7303: joint portion, 7304:display portion, 7305: display portion, 7306: speaker, 7307: recordingmedium insertion portion, 7308: LED lamp, 7309: operation key, 7310:connection terminal, 7311: sensor, 7401: housing, 7402: display portion,7403: operation button, 7404: external connection port, 7405: speaker,7406: microphone, 9033: clip, 9034: switch, 9035: power supply switch,9036: switch, 9038: operation switch, 9630: housing, 9631: displayportion, 9631 a: display portion, 9631 b: display portion, 9632 a: touchpanel region, 9632 b: touch panel region, 9633: solar cell, 9634: chargeand discharge control circuit, 9635: battery, 9636: DC-DC converter,9637: operation key, 9638: converter, 9639: button

This application is based on Japanese Patent Application serial no.2013-249486 filed with Japan Patent Office on Dec. 2, 2013, JapanesePatent Application serial no. 2014-097803 filed with Japan Patent Officeon May 9, 2014, and Japanese Patent Application serial no. 2014-180913filed with Japan Patent Office on Sep. 5, 2014, the entire contents ofwhich are hereby incorporated by reference.

The invention claimed is:
 1. A light-emitting element comprising: a pairof electrodes; and an EL layer between the pair of electrodes, whereinthe EL layer comprises a first light-emitting layer, a secondlight-emitting layer, and a first layer between the first light-emittinglayer and the second light-emitting layer, wherein an emission spectrumof the first light-emitting layer is in a shorter wavelength region thanan emission spectrum of the second light-emitting layer, wherein thefirst light-emitting layer comprises a fluorescent substance and a hostmaterial, wherein the fluorescent substance emits light in the firstlight-emitting layer, wherein the second light-emitting layer comprisesa substance capable of converting triplet excitation energy into lightemission, wherein a singlet excited level of the host material is higherthan a singlet excited level of the fluorescent substance, and wherein atriplet excited level of the host material is lower than a tripletexcited level of the fluorescent substance.
 2. The light-emittingelement according to claim 1, wherein the first layer comprises asubstance having a hole-transport property and a substance having anelectron-transport property.
 3. The light-emitting element according toclaim 2, wherein the substance having a hole-transport property and thesubstance having an electron-transport property form an exciplex.
 4. Thelight-emitting element according to claim 1, wherein a thickness of thefirst layer is greater than 0 nm and less than or equal to 20 nm.
 5. Thelight-emitting element according to claim 1, wherein a thickness of thefirst layer is greater than or equal to 1 nm and less than or equal to10 nm.
 6. The light-emitting element according to claim 2, wherein atriplet excited level of the host material is lower than a tripletexcited level of the substance having a hole-transport property and atriplet excited level of the substance having an electron-transportproperty.
 7. The light-emitting element according to claim 1, whereinthe host material is an organic compound having a condensed aromaticring skeleton.
 8. The light-emitting element according to claim 1,wherein the host material is an organic compound having an anthraceneskeleton.
 9. The light-emitting element according to claim 1, whereinthe host material is an organic compound having an anthracene skeleton,and wherein the fluorescent substance is an organic compound having apyrene skeleton.
 10. The light-emitting element according to claim 1,wherein the second light-emitting layer comprises n layers, wherein n isan integer of 2 or larger, and wherein the n layers contain n kinds ofsubstances having different emission spectra and capable of convertingtriplet excitation energy into light emission.
 11. The light-emittingelement according to claim 1, wherein the second light-emitting layercomprises a first phosphorescent substance and a second phosphorescentsubstance as the substance capable of converting triplet excitationenergy into light emission, and wherein the first phosphorescentsubstance and the second phosphorescent substance have differentemission spectra.
 12. The light-emitting element according to claim 11,wherein the first phosphorescent substance emits light in a red region,wherein the second phosphorescent substance emits light in a greenregion, and wherein the fluorescent substance emits light in a blueregion.
 13. The light-emitting element according to claim 12, whereinthe first phosphorescent substance has a peak of an emission spectrum of580 nm to 680 nm, wherein the second phosphorescent substance has a peakof an emission spectrum of 500 nm to 560 nm, and wherein the fluorescentsubstance has a peak of an emission spectrum of 400 nm to 480 nm. 14.The light-emitting element according to claim 11, wherein the secondlight-emitting layer comprises a first phosphorescent layer and a secondphosphorescent layer, wherein the first phosphorescent layer comprisesthe first phosphorescent substance, and wherein the secondphosphorescent layer comprises the second phosphorescent substance. 15.The light-emitting element according to claim 14, wherein the firstphosphorescent substance exhibits a carrier-trapping property in thefirst phosphorescent layer.
 16. The light-emitting element according toclaim 15, wherein the carrier-trapping property is an electron-trappingproperty.
 17. A light-emitting element comprising: a pair of electrodes;and an EL layer between the pair of electrodes, wherein the EL layercomprises a first light-emitting layer, a second light-emitting layer,and a first layer between the first light-emitting layer and the secondlight-emitting layer, wherein an emission spectrum of the firstlight-emitting layer is in a shorter wavelength region than an emissionspectrum of the second light-emitting layer, wherein the firstlight-emitting layer comprises a fluorescent substance and a hostmaterial, wherein the fluorescent substance emits light in the firstlight-emitting layer, wherein the fluorescent substance comprises apyrene skeleton, wherein the host material comprises a condensedaromatic ring, and wherein the second light-emitting layer comprises asubstance capable of converting triplet excitation energy into lightemission.
 18. A light-emitting element comprising: a pair of electrodes;and an EL layer between the pair of electrodes, wherein the EL layercomprises a first light-emitting layer, a second light-emitting layer,and a first layer between the first light-emitting layer and the secondlight-emitting layer, wherein an emission spectrum of the firstlight-emitting layer is in a shorter wavelength region than an emissionspectrum of the second light-emitting layer, wherein the firstlight-emitting layer comprises a fluorescent substance and a hostmaterial, wherein the fluorescent substance emits light in the firstlight-emitting layer, wherein the fluorescent substance comprises apyrene skeleton, wherein the host material comprises a condensedaromatic ring, wherein the second light-emitting layer comprises asubstance, a first organic compound, and a second organic compound and,wherein the first organic compound and the second organic compound forman exciplex.
 19. The light-emitting element according to claim 17,wherein the condensed aromatic ring skeleton comprises an anthraceneskeleton.
 20. The light-emitting element according to claim 18, whereinthe condensed aromatic ring skeleton comprises an anthracene skeleton.21. The light-emitting element according to claim 17, wherein the secondlight-emitting layer comprises a substance, a first organic compound,and a second organic compound.